Patent Publication Number: US-2021190770-A1

Title: Compositions and methods for using fixed biological samples in partition-based assays

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present invention claims the benefit of priority to U.S. Provisional Application No. 62/952,670, filed Dec. 23, 2019, and to U.S. Provisional Application No. 63/026,500, filed May 18, 2020, each of which is incorporated herein by reference in their entirety. 
    
    
     FIELD OF THE INVENTION 
     The present disclosure relates generally to compositions and methods comprising a fixed biological sample and an un-fixing agent contained in a partition. 
     BACKGROUND OF THE INVENTION 
     Biological samples containing a variety of biomolecules can be processed for various purposes, such as detection of a disease (e.g., cancer) or genotyping (e.g., species identification). Microfluidic technologies have been developed for partitioning individual biological samples (e.g., cells) into discrete partitions (e.g., wells or droplets). Each discrete partition may be isolated from other partitions (e.g., fluidically isolated in the case of droplets), enabling accurate control of respective environments in the partitions, allowing for each biological sample in a partition to be processed separately. Biological samples in the discrete partitions can be barcoded and subjected to chemical or physical processes such as heating, cooling, or chemical reactions. This allows each discrete partition to contain its own separate assay that can be qualitatively or quantitatively processed. 
     Biological samples are unstable. When a biological sample is removed from its viable niche physical decomposition begins immediately. The degree of decomposition is determined by a number of factors including time, solution buffering conditions, temperature, source (e.g. certain tissues and cells a have higher levels of endogenous RNase activity), biological stress (e.g. enzymatic tissue dissociation can activate stress response genes), and physical manipulation (e.g. pipetting, centrifuging). The degradation includes important nucleic acid molecules (e.g., RNA), proteins, as well as higher-order 3D structure of molecular complexes, whole cells, tissues, organs, and organisms. The instability of biological samples is a significant obstacle for their use with partition-based assays (e.g., droplet-based or well-based single cell assays). Sample degradation greatly limits the ability to use such assays accurately and reproducibly with a wide range of available biological samples. 
     The problem of biological sample instability can be mitigated by preserving or fixing the sample using standard biological preservation methods such as cryopreservation, dehydration (e.g., in methanol), high-salt storage (e.g., using RNAssist or RNAlater), and/or chemical fixing agents that create covalent crosslinks (e.g., paraformaldehyde or DSP). The ability to use such a fixed biological sample in an assay, particularly a single-cell assay, requires that the fixed biological sample can be rapidly and efficiently un-fixed so that the relevant assay can be carried out before sample degradation occurs. 
     SUMMARY OF THE INVENTION 
     The present disclosure provides compositions and methods that allow the use of fixed biological samples in single-cell assays, such as partition-based gene expression profiling assays. 
     In at least one embodiment, the present disclosure provides a composition comprising a fixed biological sample and an un-fixing agent provided in a discrete partition (e.g., in a well, in a droplet, or encapsulated in a discrete droplet). 
     In at least one embodiment, the present disclosure provides a method for preparing a biological sample comprising: generating a discrete partition (e.g., a well or a droplet) comprising or encapsulating a fixed biological sample and an un-fixing agent. In at least one embodiment, the method further comprises fixing the biological sample prior to generating the discrete partition. In at least one embodiment, the amount of time prior to generating the discrete partition when the biological sample is fixed is at least 1 hour, at least 2 hours, at least 6 hours, at least 12 hours, at least 24 hours, at least 1 week, at least 1 month, at least 6 months, or longer. In at least one embodiment, the method further comprises heating the discrete partition. 
     In at least one embodiment, the present disclosure provides an assay method comprising: (a) generating a discrete partition (e.g., a well or a droplet) comprising or encapsulating a fixed biological sample, an un-fixing agent, and assay reagents; and (b) detecting analytes from the reaction of the assay reagents and the un-fixed biological sample. In at least one embodiment, the method further comprises fixing the biological sample prior to generating the discrete partition. In at least one embodiment, the amount of time prior to generating the discrete partition when the biological sample is fixed is at least 1 hour, at least 2 hours, at least 6 hours, at least 12 hours, at least 24 hours, at least 1 week, at least 1 month, at least 6 months, or longer. In at least one embodiment, the method further comprises heating the discrete partition. 
     In at least one embodiment of the compositions and methods of the present disclosure, the fixed biological sample is derived from a tissue sample, a biopsy sample, or a blood sample. In at least one embodiment, the fixed biological sample is a single cell. 
     In at least one embodiment of the compositions and methods of the present disclosure, the fixed biological sample has been fixed with a fixing reagent wherein the fixing reagent is paraformaldehyde (“PFA”); optionally, wherein the fixing reagent is a PFA solution at a concentration of 1%-4% PFA. 
     In at least one embodiment of the compositions and methods of the present disclosure, the partition (e.g., a well or a droplet) further comprises a bead. In at least one embodiment, the bead contains or carries the un-fixing agent. 
     In at least one embodiment of the compositions and methods of the present disclosure, the discrete partition (e.g., a well or a droplet) further comprises assay reagents; optionally, wherein the assay reagents are contained in a bead. 
     In at least one embodiment of the compositions and methods of the present disclosure, the discrete partition (e.g., a well or a droplet) further comprises a barcode optionally, wherein the barcode is contained as part of a support (e.g., a bead). 
     In at least one embodiment of the compositions and methods of the present disclosure, the un-fixing agent is capable of removing crosslinks formed in biomolecules by fixation with an aldehyde (e.g., paraformaldehyde, glutaraldehyde), an NHS ester (e.g., N-Hydroxysuccinimide), an imidoesters, or a combination thereof; optionally, crosslinks formed in biomolecules by fixation with a paraformaldehyde (“PFA”) solution at a concentration of 1%-4% PFA. 
     In at least one embodiment of the compositions and methods of the present disclosure, the un-fixing agent comprises a compound selected from compound (1), compound (2), compound (3), compound (4), compound (5), compound (6), compound (7), compound (8), compound (9), compound (10), compound (11), compound (12), compound (13), compound (14), compound (15), or a combination thereof, which compounds are represented by structures disclosed elsewhere herein. In at least one embodiment, the un-fixing agent compound in the composition is at a concentration of about 1 mM to about 500 mM, about 50 mM to about 300 mM, or about 50 mM to about 200 mM; optionally, wherein the concentration is about 25 mM to about 200 mM. 
     In at least one embodiment, the present disclosure provides a composition comprising a compound selected from compound (8), compound (9), compound (10), and a combination thereof: 
     
       
         
         
             
             
         
       
     
     In at least one embodiment, the present disclosure provides composition comprising a fixed biological sample and a compound represented by structure (8), (9), or (10). 
     In at least one embodiment of the composition comprising compound (8), (9), or (10), the fixed biological sample is derived from a tissue sample, a biopsy sample, or a blood sample. In at least one embodiment, the composition is provided in or encapsulated in a discrete partition (e.g., a well or a droplet). In at least one embodiment, the discrete partition further comprises a bead; optionally, wherein the bead contains or carries the compound (8), (9), or (10). 
     In at least one embodiment, the present disclosure provides a kit comprising: assay reagents; and an un-fixing agent composition, wherein the composition comprises a compound represented by structure (8), (9), or (10). 
     In at least one embodiment of the kit, the un-fixing agent composition is contained in a bead. In at least one embodiment, the assay reagents are contained in a bead; wherein the assay reagents comprise a barcode. In at least one embodiment, the kit further comprises a fixing reagent; optionally, wherein the fixing reagent is paraformaldehyde; optionally, wherein the fixing reagent is a PFA solution of 1%-4% PFA. 
     In at least one embodiment, the present disclosure provides an assay method comprising: (a) incubating a fixed cell with an un-fixing solution comprising an un-fixing agent and an enzyme (e.g., a protease), thereby generating an un-fixed cell; (b) separating the un-fixed cell from the un-fixing solution; (c) combining the un-fixed cell with assay reagents; and detecting analytes from the reaction of the assay reagents. 
     In at least one embodiment of the assay method, the fixed cells have been fixed with paraformaldehyde (“PFA”); optionally, fixed with PFA at a concentration of 1%-4%. In at least one embodiment, the amount of time prior to incubating that the cells have been fixed is at least 1 hour, at least 2 hours, at least 6 hours, at least 12 hours, at least 24 hours, at least 3 days, at least 1 week, at least 1 month, at least 6 months, or longer. 
     In at least one embodiment of the assay method, the un-fixing agent is a composition comprising a compound selected from compound (1), compound (2), compound (3), compound (4), compound (5), compound (6), compound (7), compound (8), compound (9), compound (10), compound (11), compound (12), compound (13), compound (14), compound (15), or a combination thereof. In at least one embodiment, the enzyme is a protease, such as an endopeptidase, an exopeptidase, or a proteinase. In one other embodiment, the protease is a cold-active. In one embodiment, the cold-active protease is active at about 5° C., at about 10° C., about 15° C., about 20° C., about 25° C., about 30° C., about 35° C., about 40° C., or about 45° C. In another embodiment, the cold-active protease is active between about 5° C. and about 45° C., about 10° C. and about 40° C., about 15° C. and about 35° C., or about 20° C. and about 30° C. In at least one embodiment, the protease has maximum activity at a temperature of between about 50° C. and about 60° C. 
     In one other embodiment, the protease is thermolabile and is inactive at a temperature of about 50° C., about 55° C., about 60° C., about 65° C., about 70° C., about 75° C., or higher. In another embodiment, the protease is inactivated over a period of time at a particular temperature, e.g., about 50° C. for about 10 minutes (or more) or is inactivated at about 50° C., about 55° C., about 60° C., or about 65° C. over a period of time at a particular temperature, e.g., about 50° C. for about 10 minutes. In another embodiment, the protease is selected from Subtilisin A, Proteinase K, ArcticZymes Proteinase, Thermolabile Proteinase K (New England Biolabs) and any combination thereof. Protocols using cold-active protease for preparing biological samples are further described in U.S. Provisional Application No. 63/008,591 incorporated herein by reference in its entirety. In at least one embodiment, the un-fixing agent compound in the composition is at a concentration of about 1 mM to about 500 mM, about 50 mM to about 300 mM, or about 50 mM to about 200 mM; optionally, wherein the concentration is about 25 mM to about 200 mM. 
     In at least one embodiment of the assay method, incubating the fixed cell with the un-fixing solution is for 30-60 min at a temperature of from about 50 C to 60 C; optionally, wherein the incubating is for at least 45 minutes at 53 C. 
     In at least one embodiment of the assay method, incubating the fixed cell with the un-fixing solution is for 30-120 min at a temperature of from about 15 C to 60 C; optionally, wherein the incubating is for at least 90 minutes at 25 C. 
     In at least one embodiment of the assay method, the method further comprises incubating with a protease inhibitor at a temperature of from about 60 C to about 70 C for about 10 to about 20 min; optionally, wherein the protease inhibitor is PMSF at a concentration of about 1 mM. 
     In at least one embodiment of the assay method, separating the un-fixed cell from the un-fixing solution comprises centrifuging the solution to provide a pellet of un-fixed cells. In at least one embodiment, the separating further comprises resuspending the pellet of un-fixed cells in a solution. 
     In at least one embodiment of the assay method, the assay reagents comprise a reverse transcriptase; optionally, wherein the assay reagents further comprise reagents for cDNA synthesis. 
     In at least one embodiment of the assay method, combining the un-fixed cell with assay reagents further comprises generating a discrete partition (e.g., a well or a droplet) comprising or encapsulating the un-fixed cell and assay reagents; optionally, wherein the discrete partition (e.g., a well or a droplet) further comprises a barcode, whereby RNA of the un-fixed cell is labeled by the barcode. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       A better understanding of the novel features and advantages of the present disclosure will be obtained by reference to the following detailed description that sets forth illustrative embodiments, in which the principles of the disclosure are utilized, and the accompanying drawings (also “Figure” and “FIG.” herein), of which: 
         FIG. 1  shows an example of a microfluidic channel structure for partitioning individual biological particles. 
         FIG. 2  shows an example of a microfluidic channel structure for delivering barcode carrying beads to droplets. 
         FIG. 3  shows an example of a microfluidic channel structure for co-partitioning biological particles and reagents. 
         FIG. 4  shows an example of a microfluidic channel structure for the controlled partitioning of beads into discrete droplets. 
         FIG. 5  shows an example of a microfluidic channel structure for increased droplet generation throughput. 
         FIG. 6  shows another example of a microfluidic channel structure for increased droplet generation throughput. 
         FIG. 7  depicts a plot of results of transcriptomic mapping of fixed PBMC samples treated with the un-fixing agents of compounds (1), (4), (8), and (11) relative to untreated samples, as described in Example 1. 
         FIG. 8  depicts a plot of relative transcriptomic library complexity resulting from transcriptomic mapping samples treated with the un-fixing agents of compounds (1), (4), and (8), as described in Example 1. 
         FIG. 9  depicts a correlation plot of transcriptomic mapping profiles observed from fixed PBMCs treated with the un-fixing agent of compound (8) and the profiles observed from fresh PBMCs. R 2 =0.882. 
         FIG. 10  depicts a correlation plot of transcriptomic mapping profiles observed from fixed PBMCs that were not treated with an un-fixing agent and the profiles observed from fresh PBMCs. R 2 =0.658. 
         FIG. 11A ,  FIG. 11B , and  FIG. 11C  depicts cDNA electropherogram plots from single-cell 3′-RT reactions.  FIG. 11A  shows the plot for Fresh cells partitioned with single-cell 3′-RT reagent mix;  FIG. 11B  shows the plot for Fresh cells partitioned with the single-cell 3′-RT reagent mix and the protease, PK;  FIG. 11C , shows the plot for PFA-fixed cells which have undergone a bulk un-fixing treatment using compound (8) and PK, followed by pelleting and washing, before being partitioned with the single-cell 3′-RT reagent mix. 
         FIG. 12A ,  FIG. 12B , and  FIG. 12C  depicts cDNA electropherogram plots from single-cell 3′-RT reactions.  FIG. 12A : fresh PBMCs;  FIG. 12B : 4% PFA fixed PBMCs treated only with ArcticZymes Proteinase;  FIG. 12C : 4% PFA fixed PBMCs treated with ArcticZymes Proteinase and the un-fixing agent of compound (8), as described in Example 8. 
         FIG. 13  depicts plots of cell counting of different PBMC cell types found in fresh cells as compared to PFA-fixed cells subjected to the un-fixing treatment with ArcticZymes Proteinase and the un-fixing agent of compound (8), as described in Example 8. 
         FIG. 14  depicts plots of cell counting of different cell types found in fresh kidney tissue as compared to cell types found in PFA-fixed kidney tissue subjected to the un-fixing treatment with ArcticZymes Proteinase and the un-fixing agent of compound (8), as described in Example 9. 
         FIG. 15  shows an exemplary barcode carrying bead. 
         FIG. 16  shows another exemplary barcode carrying bead. 
         FIG. 17  shows an exemplary microwell array schematic. 
         FIG. 18  shows an exemplary microwell array workflow for processing nucleic acid molecules. 
         FIG. 19  schematically illustrates examples of labelling agents. 
         FIG. 20  depicts an example of a barcode carrying bead. 
         FIG. 21A ,  FIG. 21B , and  FIG. 21C  schematically depict an example workflow for processing nucleic acid molecules. 
         FIG. 22  shows gene expression correlations computed as the Pearson correlation (R 2 ) of gene expression counts between samples. 
         FIG. 23  shows PBMC cell type counts (fixed and fresh cells) determined by gene expression profiling. The 1, 2, 3 numbering for Monocytes+CD14 in the plot indicates the different experimental conditions and the same numbering scheme is used for other cell types in the same plot. 
     
    
    
     DETAILED DESCRIPTION 
     For the descriptions herein and the appended claims, the singular forms “a”, and “an” include plural referents unless the context clearly indicates otherwise. Thus, for example, reference to “a protein” includes more than one protein, and reference to “a compound” refers to more than one compound. It is further noted that the claims may be drafted to exclude any optional element. As such, this statement is intended to serve as antecedent basis for use of such exclusive terminology as “solely,” “only” and the like in connection with the recitation of claim elements, or use of a “negative” limitation. The use of “comprise,” “comprises,” “comprising” “include,” “includes,” and “including” are interchangeable and not intended to be limiting. It is to be further understood that where descriptions of various embodiments use the term “comprising,” those skilled in the art would understand that in some specific instances, an embodiment can be alternatively described using language “consisting essentially of” or “consisting of.” 
     Where a range of values is provided, unless the context clearly dictates otherwise, it is understood that each intervening integer of the value, and each tenth of each intervening integer of the value, unless the context clearly dictates otherwise, between the upper and lower limit of that range, and any other stated or intervening value in that stated range, is encompassed within the invention. The upper and lower limits of these smaller ranges may independently be included in the smaller ranges, and are also encompassed within the invention, subject to any specifically excluded limit in the stated range. Where the stated range includes one or both of these limits, ranges excluding (i) either or (ii) both of those included limits are also included in the invention. For example, “1 to 50,” includes “2 to 25,” “5 to 20,” “25 to 50,” “1 to 10,” etc. 
     Generally, the nomenclature used herein and the techniques and procedures described herein include those that are well understood and commonly employed by those of ordinary skill in the art, such as the common techniques and methodologies described in e.g., Green and Sambrook,  Molecular Cloning: A Laboratory Manual  (Fourth Edition), Vols. 1-3, Cold Spring Harbor Laboratory, Cold Spring Harbor, N.Y., 2012 (hereinafter “Sambrook”); and  Current Protocols in Molecular Biology , F. M. Ausubel et al., eds., originally published in 1987 in book form by Greene Publishing Associates, Inc. and John Wiley &amp; Sons, Inc., and regularly supplemented through 2011, and now available in journal format online as  Current Protocols in Molecular Biology , Vols. 00-130, (1987-2020), published by Wiley &amp; Sons, Inc. in the Wiley Online Library (hereinafter “Ausubel”). 
     All publications, patents, patent applications, and other documents referenced in this disclosure are hereby incorporated by reference in their entireties for all purposes to the same extent as if each individual publication, patent, patent application or other document were individually indicated to be incorporated by reference herein for all purposes. 
     Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the present invention pertains. It is to be understood that the terminology used herein is for describing particular embodiments only and is not intended to be limiting. For purposes of interpreting this disclosure, the following description of terms will apply and, where appropriate, a term used in the singular form will also include the plural form and vice versa. 
     A. Use of Fixing Reagents to Stabilize Biological Samples 
     The main methods for preserving biological sample integrity, and limiting decomposition include cryopreservation, dehydration (e.g., methanol), high salt storage (e.g., using RNAssist, or RNAlater), and treatment with chemical fixing agents that typically create covalently crosslinks in the biomolecules of the sample (e.g., paraformaldehyde). These techniques for stabilizing biological samples can be used alone or in combination, and each can be reversed to various extents using various un-fixing treatments. 
     Recognized herein is the need for methods, compositions, kits, and systems for analyzing multiple cellular analytes (e.g., genomic, epigenomic, transcriptomic, metabolomic, and/or proteomic information) from fixed biological samples, e.g., individual cells, a population of cells, tissue samples, and other kinds of biological samples. The ability to use a fixed biological sample in a partition-based assay (e.g., a single cell assay), however, requires rapid and efficient un-fixing of the sample to obtain access to the relevant cellular analytes for processing before degradation occurs. Ideally, the assay data obtained from an un-fixed biological sample should be identical to that obtained from a fresh sample, or resemble a sample obtained from its natural environment as closely as possible. 
     The present invention provides methods, composition, kits, and systems for treating fixed biological samples in order to process cellular analytes. Cellular analytes that are suitable for use with the present invention include, without limitation, intracellular and partially intracellular analytes. The cellular analyte may be a protein, a metabolite, a metabolic byproduct, an antibody or antibody fragment, an enzyme, an antigen, a carbohydrate, a lipid, a macromolecule, or a combination thereof (e.g., proteoglycan) or other biomolecule. The cellular analyte may be a nucleic acid molecule. The cellular analyte may be a deoxyribonucleic acid (DNA) molecule or a ribonucleic acid (RNA) molecule. The DNA molecule may be a genomic DNA molecule. The cellular analyte may comprise coding or non-coding RNA. The RNA may be messenger RNA (mRNA), ribosomal RNA (rRNA) or transfer RNA (tRNA), for example. The RNA may be a transcript. The RNA may be small RNA that are less than 200 nucleic acid bases in length, or large RNA that are greater than 200 nucleic acid bases in length. Small RNAs may include 5.8S ribosomal RNA (rRNA), 5S rRNA, transfer RNA (tRNA), microRNA (miRNA), small interfering RNA (siRNA), small nucleolar RNA (snoRNAs), Piwi-interacting RNA (piRNA), tRNA-derived small RNA (tsRNA) and small rDNA-derived RNA (srRNA). The RNA may be double-stranded RNA or single-stranded RNA. The RNA may be circular RNA. 
     In some instances, the cellular analyte is associated with an intermediary entity, wherein the intermediary entity is analyzed to provide information about the cellular analyte and/or the intermediary entity itself. For instance, an intermediary entity (e.g., an antibody) may be bound to a partially intracellular analyte (e.g., a cell surface receptor), where the intermediary entity is processed to provide information about the intermediary entity, the partially intracellular analyte, or both. In one embodiment, the intermediary entity comprises an identifier (e.g., a barcode molecule) that can be used to generate barcode molecules (e.g., droplet-based barcoding) as further described herein. 
     The term “partition,” as used herein, generally, refers to a space or volume that may be suitable to contain one or more species or conduct one or more reactions. A partition may be a physical compartment, such as a droplet or well (e.g., a microwell). The partition may isolate space or volume from another space or volume. The droplet may be a first phase (e.g., aqueous phase) in a second phase (e.g., oil) immiscible with the first phase. The droplet may be a first phase in a second phase that does not phase separate from the first phase, such as, for example, a capsule or liposome in an aqueous phase. A partition may comprise one or more other (inner) partitions. In some cases, a partition may be a virtual compartment that can be defined and identified by an index (e.g., indexed libraries) across multiple and/or remote physical compartments. For example, a physical compartment may comprise a plurality of virtual compartments. 
     Droplet-based assays typically involve a biological sample isolated and partitioned as single cells in discrete droplets in an emulsion. The discrete droplet usually also includes a unique identifier for the sample in the form of a unique oligonucleotide sequence also contained in the discrete droplet. The discrete droplet can also contain the assay reagents that are used to generate detectable analytes (e.g.,  3 ′ cDNA sequences) from the sample and provide useful information about it (e.g., RNA transcript profile). Further details of methods and compositions for carrying out droplet-based assays are provided elsewhere herein. 
     Preparation of the partition containing a biological sample that is useful in a partition-based assay involves numerous steps (e.g., sample transport, tissue dissociation, liquid phase washing and transfer, library preparation) that typically take from a few hours to days. During this preparation time an un-fixed biological sample will begin to degrade, and decompose resulting in significant loss of sample quality and potentially leading to assay results that do not reflect the natural state of the sample. 
     The present disclosure provides compositions and methods for preparing a fixed biological sample that maintain its integrity from the biological point of collection, but which fixed biological sample is capable of being provided or encapsulated (e.g., in a discrete droplet or a discrete well) with an un-fixing agent, undergo un-fixing to generate a partitioned, (e.g., encapsulated) un-fixed biological sample capable of undergoing a partition-based assay. 
     As provided in greater detail elsewhere herein, in at least one embodiment, the composition comprises a fixed biological sample and an un-fixing agent in a discrete partition, e.g., provided in or encapsulated in a discrete droplet. In at least one embodiment, the method for preparing such a composition comprises: providing a discrete partition (e.g., a well or a droplet) comprising a fixed biological sample and an un-fixing agent (e.g., generating a discrete droplet comprising or encapsulating a fixed biological sample and an un-fixing agent); optionally, wherein the method comprises fixing the biological sample prior to partitioning in a discrete partition (e.g., generating the discrete droplet). In another embodiment, the present disclosure provides an assay method, wherein the method comprises (a) providing a discrete partition comprising a fixed biological sample, an un-fixing agent, and assay reagents (e.g., generating a discrete droplet comprising or encapsulating a fixed biological sample, an un-fixing agent, and assay reagents); and (b) detecting analytes from the reaction of the assay reagents and the un-fixed biological sample. 
     These compositions and methods as disclosed herein allow for the use of fixed biological samples derived from a tissue sample, a biopsy sample, or a blood sample, that have been fixed with paraformaldehyde, and can comprise a fixed biological sample of a single cell. The stabilizing effect of the fixatives and the efficient of the un-fixing agents disclosed herein allow for the amount of time of sample fixation prior to providing the discrete partition (e.g., generating the discrete droplet) to be at least 1 hour, at least 2 hours, at least 6 hours, at least 12 hours, at least 24 hours, at least 1 week, at least 1 month, at least 6 months, or longer. 
     As noted above, the compositions and methods of the present disclosure can allow for the use of a wide range of previously fixed biological samples in single-cell partition-based assays (e.g., droplet-based or well-based assays). The term “biological sample,” as used herein refers to any sample of biological origin that includes a biomolecule, such as a nucleic acid, a protein, a carbohydrate, and/or a lipid. Biological samples used in the methods and compositions of the disclosure include blood and other liquid samples of biological origin, solid tissue samples such as a tissue sample (i.e., tissue specimen), a biopsy (i.e., a biopsy specimen), or tissue cultures or cells derived therefrom and the progeny thereof. This includes samples that have been manipulated in any way after isolation from the biological source, such as by treatment with reagents (e.g., fixation reagents, thereby generating a fixed biological sample); samples such as tissues that are embedded in medium (e.g., paraffin); sectioned tissue sample (e.g., sectioned samples that are mounted on a solid substrate such as a glass slide); washed; or enrichment for certain cell populations, such as cancer cells, neurons, stem cells, etc. The term also encompasses samples that have been enriched for particular types of molecules, e.g., nucleic acids, polypeptides, etc. “Biological sample” encompasses a clinical sample, and also includes tissue obtained by surgical resection, tissue obtained by biopsy, cells in culture, cell supernatants, cell lysates, tissue samples (i.e., tissue specimens), organs, bone marrow, blood, plasma, serum, and the like. A “biological sample” also includes a sample obtained from a patient&#39;s cancer cell, e.g., a sample comprising polynucleotides and/or polypeptides that is obtained from a patient&#39;s cancer cell (e.g., a cell lysate or other cell extract comprising polynucleotides and/or polypeptides); and a sample having cells (e.g., cancer cells) from a patient. 
     It is contemplated that the biological samples used in the compositions and methods of the present disclosure can be derived from another sample. Biological samples can include a tissue sample, such as a biopsy, core biopsy, needle aspirate, or fine needle aspirate. Biological samples also include a biological fluid sample, such as a blood sample, urine sample, or saliva sample, or the biological sample may be a skin sample, a cheek swab. The biological sample may be a plasma or serum sample. The biological sample may include cells or be a cell-free sample. A cell-free sample may include extracellular polynucleotides. Extracellular polynucleotides may be isolated from a bodily sample that may be selected from the group consisting of blood, plasma, serum, urine, saliva, mucosal excretions, sputum, stool and tears. 
     The ability to use a fixed biological sample in a partition-based assay system (e.g., a droplet-based or a well-based assay system) is a feature of the compositions and methods of the present disclosure. The term “fixed” as used herein with regard to biological samples refers the state of being preserved from decay and/or degradation. “Fixation” refers to a process that results in a fixed sample, and can include contacting the biomolecules within a biological sample with a fixative (or fixation reagent) for some amount of time, whereby the fixative results in covalent bonding interactions such as crosslinks between biomolecules in the sample. A “fixed biological sample” refers to a biological sample that has been contacted with a fixation reagent or fixative. For example, a formaldehyde-fixed biological sample has been contacted with the fixation reagent formaldehyde. “Fixed cells” or “fixed tissues” refer to cells or tissues that have been in contact with a fixative under conditions sufficient to allow or result in the formation of intra- and inter-molecular covalent crosslinks between biomolecules in the biological sample. 
     Herein, “un-fixed” refers to the processed condition of a cell, a plurality of cells, a tissue sample or any other biological sample that is characterized by a prior state of fixation followed by a reversal of the prior state of fixation. For instance, an un-fixed cell may also be referred to as a “previously fixed” cell. In one embodiment, an un-fixed cell is characterized by broken or reversed covalent bonds in the biomolecules of the cell(s) or sample, where such covalent bonds were previously formed by treatment with a fixation agent (e.g., paraformaldehyde or PFA). 
     In one embodiment, the methods, compositions, systems, and kits described herein provide un-fixed cells (e.g., from previously fixed cells) that following treatment with an un-fixing agent (i) remain intact and/or (ii) retain cellular analytes. In another embodiment, the intact un-fixed cells retain sufficient amounts of cellular analytes, e.g., nucleic acid analytes, for downstream processing, e.g., partition-based barcoding. In other embodiments, the compositions, systems, and kits described herein allow for the preparation of a plurality of un-fixed cells, wherein an un-fixed cell from said plurality of un-fixed cells retains a plurality of cellular analytes, e.g., nucleic acid analytes. In an additional embodiment, the un-fixed cell can be further processed, e.g., by partition-based barcoding methods, to provide barcoded nucleic acid molecules that comprise sequences corresponding to cellular analytes of said plurality of cellular analytes from said unfixed cell. 
     In one aspect, the present invention provides a method for analysis of fixed cells. In one embodiment, the method comprises providing a plurality of fixed cells, wherein a fixed cell of said plurality of fixed cells comprises a plurality of crosslinked nucleic acid molecules. In another embodiment, the method further comprises un-fixing said fixed cell with an un-fixing agent to provide an un-fixed cell comprising a plurality of un-crosslinked nucleic acid molecules from said plurality of crosslinked nucleic acid molecules. In other embodiments, said plurality of crosslinked nucleic acid molecules comprises cross-linked ribonucleic acid (RNA) molecules and/or said plurality of un-crosslinked nucleic acid molecules comprises un-crosslinked ribonucleic acid (RNA) molecules. 
     In one additional embodiment, the method further comprises generating a plurality of barcoded nucleic acid molecules from said plurality of un-crosslinked nucleic acid molecules and a plurality of nucleic acid barcode molecules. In another embodiment, the generating is performed in a plurality of partitions. In one other embodiment, the plurality of partitions is a plurality of droplets or a plurality of wells. In another embodiment, a barcoded nucleic acid molecule of said plurality of barcoded nucleic acid molecules comprises i) a sequence corresponding to an un-crosslinked nucleic acid molecule of said plurality of said un-crosslinked nucleic acid molecules or complement thereof, and ii) a barcode sequence or complement thereof. In one embodiment, said sequence corresponding to an un-crosslinked nucleic acid molecule is a sequence corresponding to an un-crosslinked RNA molecule. In other embodiments, the barcode sequence is a partition-specific barcode sequence. In another embodiment, a partition of said plurality of partitions comprises said un-fixed cell and a support comprising said plurality of nucleic acid barcode molecules. In other embodiments, the support is a bead (e.g., a gel bead). 
     In other embodiments, said plurality of fixed cells is a plurality of paraformaldehyde fixed cells, said un-fixing agent is capable of removing crosslinks formed in biomolecules by fixation with an aldehyde, an NHS ester (e.g.; N-Hydroxysuccinimide), an imidoesters, or a combination thereof, and/or said un-fixing agent is capable of removing crosslinks formed in biomolecules by fixation with paraformaldehyde. 
     In another embodiment, said un-fixing agent comprises a compound selected from compound (8), compound (1), compound (2), compound (3), compound (4), compound (5), compound (6), compound (7), compound (9), compound (10), compound (11), compound (12), compound (13), compound (14), compound (15), and a combination thereof, as further described herein. In other embodiments, said un-fixing agent comprises more than one component and is provided as separate components in separate compositions or as part of one composition. In one additional embodiment, said un-fixing agent further comprises a protease, which can optionally be a thermolabile and/or cold-active protease. 
     In an additional embodiment, said fixed cell comprises a labeling agent. In one other embodiment, said labeling agent is selected from the group consisting of protein, a peptide, an antibody, a lipophilic moiety, a cell surface receptor binding molecule, a receptor ligand, a small molecule, a bi-specific antibody, a bi-specific T-cell engager, a T-cell receptor engager, a B-cell receptor engager, a pro-body, an aptamer, a monobody, an affimer, a darpin, a protein scaffold, and any combination thereof. In other embodiments, the labeling agent comprises a reporter oligonucleotide. In one embodiment, the reporter oligonucleotide comprises a reporter sequence that identifies the labeling agent. In other embodiments, the method further comprises generating an additional barcoded nucleic acid molecule from said reporter oligonucleotide and said plurality of nucleic acid barcode molecules, wherein said additional barcoded nucleic acid molecule of said plurality of barcoded nucleic acid molecules comprises i) said reporter sequence or a complement thereof and ii) said barcode sequence of complement thereof. 
     The amount of time a biological sample is contacted with a fixative to provide a fixed biological sample depend on the temperature, the nature of the sample, and the fixative used. For example, a biological sample can be contacted by a fixation reagent for 72 or less hours (e.g., 48 or less hours, 24 or less hours, 18 or less hours, 12 or less hours, 8 or less hours, 6 or less hours, 4 or less hours, 2 or less hours, 60 or less minutes, 45 or less minutes, 30 or less minutes, 25 or less minutes, 20 or less minutes, 15 or less minutes, 10 or less minutes, 5 or less minutes, or 2 or less minutes). 
     Generally, contact of biological sample (e.g., a cell) with a fixation reagent (e.g., paraformaldehyde or PFA) results the formation of intra- and inter-molecular covalent crosslinks between biomolecules in the biological sample. In some cases, the fixation reagent, formaldehyde, is known to result in covalent aminal crosslinks within RNA, DNA, and/or protein molecules. Examples of fixation reagents include but are not limited to aldehyde fixatives (e.g., formaldehyde, also commonly referred to as “paraformaldehyde,” “PFA,” and “formalin”; glutaraldehyde; etc.), imidoesters, NHS (N-Hydroxysuccinimide) esters, and the like. 
     The formation of crosslinks in biomolecules (e.g., proteins, RNA, DNA) due to fixation greatly reduces the ability to detect (e.g., bind to, amplify, sequence, hybridize to) the biomolecules in standard assay methods, Common techniques to remove the crosslinks induced by fixative reagents (e.g., heat, acid) can cause further damage to the biomolecules (e.g., loss of bases, chain hydrolysis, cleavage, denaturation, etc.). Further description of the consequences of fixation of tissue samples and the benefits of removing adducts and/or crosslinks are described in U.S. Pat. No. 8,288,122, which is hereby incorporated by reference in its entirety. For example, the widely used fixative reagent, paraformaldehyde or PFA, fixes tissue samples by catalyzing crosslink formation between basic amino acids in proteins, such as lysine and glutamine. Both intra-molecular and inter-molecular crosslinks can form in the protein. These crosslinks can preserve protein secondary structure and also eliminate enzymatic activity in the preserved tissue sample. 
     In some embodiments, the fixative or fixation reagent useful in the methods of the present disclosure is formaldehyde. The term “formaldehyde” when used in the context of a fixative also refers “paraformaldehyde” (or “PFA”) and “formalin”, both of which are terms with specific meanings related to the formaldehyde composition (e.g., formalin is a mixture of formaldehyde and methanol). Thus, a formaldehyde-fixed biological sample may also be referred to as formalin-fixed or PFA-fixed. Protocols and methods for the use of formaldehyde as a fixation reagent to prepare fixed biological samples are well known in the art, and can be used in the methods and compositions of the present disclosure. For example, suitable ranges of formaldehyde concentrations for use in preparing a fixed biological sample is 0.1 to 10%, 1-8%, 1-4%, 1-2%, 3-5%, or 3.5-4.5%. In some embodiments of the present disclosure the biological sample is fixed using a final concentration of 1% formaldehyde, 4% formaldehyde, or 10% formaldehyde. Typically, the formaldehyde is diluted from a more concentrated stock solution—e.g., a 35%, 25%, 15%, 10%. 5% PFA stock solution. 
     It is contemplated that more than one fixation reagent can be used in combination in preparing a fixed biological sample. For example, in some cases biomolecules (e.g., biological samples such as tissue specimens) are contacted with a fixation reagent containing both formaldehyde and glutaraldehyde, and thus the contacted biomolecules can include fixation crosslinks resulting both from formaldehyde induced fixation and glutaraldehyde induced fixation. Typically, a suitable concentration of glutaraldehyde for use as a fixation reagent is 0.1 to 1%. 
     B. Use of Un-Fixing Agents with Fixed Biological Samples 
     Conditions for reversing the effects of fixing a biological sample are known in the art, however, these conditions tend to be harsh. See e.g., WO2001/46402; US2005/0014203A1, and US2009/0202998A1. For example, treatment of PFA-treated tissue samples includes heating to 60-70 C in Tris buffer for several hours, and yet typically results in removal of only a fraction of the fixative-induced crosslinks. Furthermore, the harsh un-fixing treatment conditions can result in permanent damage to biomolecules, particularly nucleic acids, in the sample. Recently, less harsh un-fixing techniques and conditions have been proposed that utilize compounds capable of chemically reversing the crosslinks resulting from fixation. See e.g., Karmakar et al., “Organocatalytic removal of formaldehyde adducts from RNA and DNA bases,” Nature Chemistry, 7: 752-758 (2015); US 2017/0283860A1; and US 2019/0135774A1. 
     The terms “un-fixing agent” (or “de-crosslinking agent”) as used herein refer to a compound or composition that reverses fixation and/or removes the crosslinks within or between biomolecules in a sample caused by previous use of a fixation reagent. In some embodiments, un-fixing agents are compounds that act catalytically in removing crosslinks in a fixed sample. Exemplary compounds (1)-(15) useful as un-fixing agents in the methods and compositions of the present disclosure include the compounds of Table 1 below. 
     
       
         
           
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Exemplary Un-fixing Agent Compounds 
               
               
                   
               
             
            
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 (1) 
               
               
                 2-amino-5-methylbenzoic acid 
               
               
                 (CAS No. 2941-78-8; Sigma-Aldrich) 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 (2) 
               
               
                 2-amino-5-nitrobenzoic acid 
               
               
                 (CAS No. 616-79-5; Sigma-Aldrich) 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 (3) 
               
               
                 (2-amino-5-methylphenyl)phosphonic acid 
               
               
                 (CAS 69675-98-5; Ambeed Inc.) 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 (4) 
               
               
                 2-amino-5-methylbenzenesulfonic acid 
               
               
                 (CAS No. 88-44-8; Sigma-Aldrich) 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 (5) 
               
               
                 2,5-diaminobenzenesulfonic acid 
               
               
                 (CAS No. 88-45-9; Sigma-Aldrich) 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 (6) 
               
               
                 2-amino-3,5-dimethylbenzenesulfonic acid 
               
               
                 (CAS No. 88-22-2; TCI Co. Ltd., Tokyo, JP) 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 (7) 
               
               
                 (2-amino-5-nitrophenyl)phosphonic acid 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 (8) 
               
               
                 (4-aminopyridin-3-yl)phosphonic acid 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 (9) 
               
               
                 (3-aminopyridin-2-yl)phosphonic acid 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 (10) 
               
               
                 (5-aminopyrimidin-4-yl)phosphonic acid 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 (11) 
               
               
                 (2-amino-5-{[2-(2-poly-ethoxy)ethyl]carbamoyl}phenyhphosphonic acid 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 (12) 
               
               
                 (2S,4R)-4-hydroxypyrrolidine-2-carboxylic acid 
               
               
                 (“trans-4-hydroxy-L-proline;” CAS No. 51-35-4; Sigma-Aldrich) 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 (13) 
               
               
                 (2S,4R)-4-aminopyrrolidine-2-carboxylic acid 
               
               
                 (“trans-4-aminoproline;” CAS No. 16257-88-8) 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 (14) 
               
               
                 (2S,4S)-4-[(pyridin-4-yl)oxy]pyrrolidine-2-carboxylic acid 
               
               
                 (CAS No. 2309431-82-9; Enamine Ltd.) 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 (15) 
               
               
                 (2S,4S)-4-[(pyridin-3-yl)oxy]pyrrolidine-2-carboxylic acid 
               
               
                 (“cis-m-O-Py-Pro”) 
               
               
                   
               
            
           
         
       
     
     At least one of the un-fixing agents of Table 1, compound (3), has previously been shown to catalytically break down the aminal and hemi-aminal adducts that form in RNA treated with formaldehyde, and are compatible with many RNA extraction and detection conditions. See e.g., Karmakar et al., “Organocatalytic removal of formaldehyde adducts from RNA and DNA bases,” Nature Chemistry, 7: 752-758 (2015); and US 201710283860A1. 
     Proline is a unique amino acid that contains a secondary amine in a 5-membered ring, resulting in high nucleophilicity. The high nucleophilicity together with a proximal amine or acid moiety in the proline analog structures of compounds (12), (13), (14), and (15) suggests that these compounds, like the compounds (1)-(11), also can be used as catalytically break down the aminal and hemi-aminal adducts that form in formaldehyde-fixed RNA and other biomolecules. 
     Compounds (1)-(6), (12), and (14) are commercially available. The compounds (7), (8), (9), (10), (11), (13), and (15) can be prepared from commercially available reagents using standard chemical synthesis techniques well-known in the art. See e.g., Crisalli et al., “Importance of ortho Proton Donors in Catalysis of Hydrazone Formation,”  Org. Lett.  2013, 15, 7, 1646-1649. 
     Compounds (8) and (11) can be prepare by 2-step and 4-step syntheses, respectively, as described in Example 1. Briefly, in preparing compound (8), the compound, diethyl (4-aminopyridin-3-yl)phosphonate is prepared according to the procedure described in Guilard, R. et al.  Synthesis,  2008, 10, 1575-1579. Then, the target compound (8), (4-aminopyridin-3-yl)phosphonic acid) is prepared by acid hydrolysis of the precursor compound of the diethyl (4-aminopyridin-3-yl)phosphonate. Compounds (9) and (10) can be prepared from similarly straightforward procedures. For example, compound (9) can be prepared in 2-steps from 2-bromopyridin-3-amine (CAS Reg. #39856-58-1; Sigma-Aldrich, St. Louis, Mo.) as shown in the scheme below. 
     
       
         
         
             
             
         
       
     
     Compound (10) is prepared similarly in 2-steps from 4-bromopyrimidin-5-amine (CAS Reg. #849353-34-0; Ambeed, Inc., Arlington Heights, Ill., USA) as shown in the scheme below. 
     
       
         
         
             
             
         
       
     
     The proline analog compounds (13) and (15) are prepared via a straightforward single step deprotection from commercially available protected precursor compounds as described in Example 10. 
     Accordingly, in some embodiments of the compositions and methods of the present disclosure, the un-fixing agent used in the composition or method can comprise a compound selected from Table 1. For example, the un-fixing agent can comprise a compound of any of compound (1), compound (2), compound (3), compound (4), compound (5), compound (6), compound (7), compound (8), compound (9), compound (10), compound (11), compound (12), compound (13), compound (14), compound (15), or a combination of one or more the compounds of Table 1. 
     At least three un-fixing agents for Table 1 (i.e., the compounds represented by structures (8), (9), or (10)) appear to be novel compound structures. Accordingly, in at least one embodiment, the present disclosure provides a composition comprising a compound selected from compound (8), compound (9), compound (10), and a combination thereof. Furthermore, in view of the ability to use these compounds as un-fixing agent, in at least one embodiment, the present disclosure provides novel compositions comprising a fixed biological sample and a compound selected from compounds (8), (9), (10), or a combination thereof. Additionally, it is contemplated that in some embodiments, the composition comprising a compound (8), (9), (10), or a combination thereof, the fixed biological sample is derived from a tissue sample, a biopsy sample, or a blood sample. In at least one embodiment, the composition is provided in or encapsulated in a discrete partition (e.g., a well or a droplet). In at least one embodiment, the discrete partition further comprises a bead; optionally, wherein the bead contains or carries a compound (8), (9), (10), or a combination thereof. 
     C. Partitioning of Fixed Biological Samples and Un-fixing Agents in Discrete Partitions 
     The compositions and methods of the present disclosure are useful to prepare fixed biological samples that are partitioned in discrete partitions along with an un-fixing agent capable of reversing the fixed state of the biomolecules in the sample while it is sequestered in the partition. In one embodiment, the fixed biological samples are provided in or encapsulated in discrete droplets along with an un-fixing agent capable of reversing the fixed state of the biomolecules in the sample while it is sequestered in the droplet. Accordingly, in some embodiments, the present disclosure provides a method for preparing a biological sample comprising: providing a discrete partition comprising a fixed biological sample and an un-fixing agent. In one embodiment, the method comprises: generating a discrete droplet comprising or encapsulating a fixed biological sample and an un-fixing agent. This method can further comprise a step of fixing the biological sample prior to providing the discrete partition (e.g., generating the discrete droplet). 
     Methods, techniques, and protocols useful for partitioning biological samples (e.g., individual cells, biomolecular contents of cells, etc.) into discrete droplets are described in the art. The discrete droplets generated act a nanoliter-scale container that can maintain separation the droplet contents from the contents of other droplets in the emulsion. Methods and systems for creating stable discrete droplets comprising or encapsulating individual particles from biological samples in non-aqueous or oil emulsions are described in, e.g., U.S. Patent Application Publication Nos. 2010/0105112 and 2019/0100632, each of which is entirely incorporated herein by reference for all purposes. 
     Briefly, discrete droplets in an emulsion comprising or encapsulating a biological sample is accomplished by introducing a flowing stream of an aqueous fluid containing the biological sample into a flowing stream of a non-aqueous fluid with which it is immiscible, such that droplets are generated at the junction of the two streams (see e.g.,  FIGS. 1-3 ). By providing the aqueous stream at a certain concentration and/or flow rate of the biological sample, the occupancy of the resulting droplets can be controlled. For example, the relative flow rates of the immiscible fluids can be selected such that, on average, the discrete droplet each contains less than one biological particle. Such a flow rate ensures that the droplets that are occupied are primarily occupied by a single sample (e.g., a single cell). Discrete droplets in an emulsion comprising or encapsulating a biological sample is also accomplished using a microfluidic architecture comprising a channel segment having a channel junction with a reservoir (see e.g.,  FIGS. 4-6 ). 
     The term “biological particle,” as used herein, generally refers to a discrete biological system derived from a biological sample. The biological particle may be a macromolecule. The biological particle may be a small molecule. The biological particle may be a virus. The biological particle may be a cell or derivative of a cell. The biological particle may be an organelle. The biological particle may be a rare cell from a population of cells. The biological particle may be any type of cell, including without limitation prokaryotic cells, eukaryotic cells, bacterial, fungal, plant, mammalian, or other animal cell type, mycoplasmas, normal tissue cells, tumor cells, or any other cell type, whether derived from single cell or multicellular organisms. The biological particle may be a constituent of a cell. The biological particle may be or may include DNA, RNA, organelles, proteins, or any combination thereof. The biological particle may be obtained from a tissue of a subject. The biological particle may be a hardened cell. Such hardened cell may or may not include a cell wall or cell membrane. The biological particle may include one or more constituents of a cell, but may not include other constituents of the cell. An example of such constituents is a nucleus or an organelle. 
     In some cases, the droplets among a plurality of discrete droplets formed in the manner contain at most one particle (e.g., one bead, one cell). The flows and microfluidic channel architectures also can be controlled to ensure a given number of singly occupied droplets, less than a certain level of unoccupied droplets, and/or less than a certain level of multiply occupied droplets. 
     In another aspect of the invention, fixed cells and un-fixing agents may then be partitioned (e.g., in a droplet or well) with other reagents for processing of one or more analytes as described herein. In one embodiment, the fixed cell and un-fixing agent may be partitioned with a support (e.g., a bead) comprising nucleic acid molecules suitable for barcoding of the one or more analytes. In another embodiment, the nucleic acid molecules may include nucleic acid sequences that provide identifying information, e.g., barcode sequence(s). 
     The term “barcode,” as used herein, generally refers to a label, or identifier, that conveys or is capable of conveying information about an analyte. A barcode can be part of an analyte. A barcode can be independent of an analyte. A barcode can be a tag attached to an analyte (e.g., nucleic acid molecule) or a combination of the tag in addition to an endogenous characteristic of the analyte (e.g., size of the analyte or end sequence(s)). A barcode may be unique. Barcodes can have a variety of different formats. For example, barcodes can include polynucleotide barcodes; random nucleic acid and/or amino acid sequences; and synthetic nucleic acid and/or amino acid sequences. A barcode can be attached to an analyte in a reversible or irreversible manner. A barcode can be added to, for example, a fragment of a deoxyribonucleic acid (DNA) or ribonucleic acid (RNA) sample before, during, and/or after sequencing of the sample. Barcodes can allow for identification and/or quantification of individual sequencing-reads. 
     As used herein, the term “barcoded nucleic acid molecule” generally refers to a nucleic acid molecule that results from, for example, the processing of a nucleic acid barcode molecule with a nucleic acid sequence (e.g., nucleic acid sequence complementary to a nucleic acid primer sequence encompassed by the nucleic acid barcode molecule). The nucleic acid sequence may be a targeted sequence (e.g., targeted by a primer sequence) or a non-targeted sequence. For example, in the methods, compositions, kits, and systems described herein, hybridization and reverse transcription of the nucleic acid molecule (e.g., a messenger RNA (mRNA) molecule) of a cell with a nucleic acid barcode molecule (e.g., a nucleic acid barcode molecule containing a barcode sequence and a nucleic acid primer sequence complementary to a nucleic acid sequence of the mRNA molecule) results in a barcoded nucleic acid molecule that has a sequence corresponding to the nucleic acid sequence of the mRNA and the barcode sequence (or a reverse complement thereof). A barcoded nucleic acid molecule may serve as a template, such as a template polynucleotide, that can be further processed (e.g., amplified) and sequenced to obtain the target nucleic acid sequence. For example, in the methods and systems described herein, a barcoded nucleic acid molecule may be further processed (e.g., amplified) and sequenced to obtain the nucleic acid sequence of the mRNA. 
     The term “bead,” as used herein, generally refers to a particle. The bead may be a solid or semi-solid particle. The bead may be a gel bead. The gel bead may include a polymer matrix (e.g., matrix formed by polymerization or cross-linking). The polymer matrix may include one or more polymers (e.g., polymers having different functional groups or repeat units). Polymers in the polymer matrix may be randomly arranged, such as in random copolymers, and/or have ordered structures, such as in block copolymers. Cross-linking can be via covalent, ionic, or inductive, interactions, or physical entanglement. The bead may be a macromolecule. The bead may be formed of nucleic acid molecules bound together. The bead may be formed via covalent or non-covalent assembly of molecules (e.g., macromolecules), such as monomers or polymers. Such polymers or monomers may be natural or synthetic. Such polymers or monomers may be or include, for example, nucleic acid molecules (e.g., DNA or RNA). The bead may be formed of a polymeric material. The bead may be magnetic or non-magnetic. The bead may be rigid. The bead may be flexible and/or compressible. The bead may be disruptable or dissolvable. The bead may be a solid particle (e.g., a metal-based particle including but not limited to iron oxide, gold or silver) covered with a coating comprising one or more polymers. Such coating may be disruptable or dissolvable. 
       FIG. 1  shows an exemplary microfluidic channel structure  100  useful for generating discrete droplets comprising or encapsulating a biological particle from a biological sample, such as a single cell. The channel structure  100  can include channel segments  102 ,  104 ,  106  and  108  communicating at a channel junction  110 . In operation, a first aqueous fluid  112  that that includes suspended particles (e.g., cells) from a biological sample  114  are transported along channel segment  102  into junction  110 , while a second fluid  116  (or “partitioning fluid”) that is immiscible with the aqueous fluid  112  is delivered to the junction  110  from each of channel segments  104  and  106  to create discrete droplets  118 ,  120  of the first aqueous fluid  112  flowing into channel segment  108 , and flowing away from junction  110 . The channel segment  108  may be fluidically coupled to an outlet reservoir where the discrete droplets can be stored and/or harvested. A discrete droplet generated may include an individual particle from a biological sample  114  (such as droplet  118 ), or discrete droplet can be generated that includes more than one particle  114  (not shown in  FIG. 1 ). A discrete droplet may contain no biological particle  114  (such as droplet  120 ). Each discrete droplet is capable of maintaining separation of its own contents (e.g., individual biological sample particle  114 ) from the contents of other droplets. 
     Typically, the second fluid  116  comprises an oil, such as a fluorinated oil, that includes a fluoro-surfactant that helps to stabilize the resulting droplets. Examples of useful partitioning fluids and fluoro-surfactants are described in e.g., U.S. Patent Application Publication No. 2010/0105112, which is entirely incorporated herein by reference for all purposes. 
     The microfluidic channels for generating discrete droplets as exemplified in  FIG. 1  may be coupled to any of a variety of different fluid sources or receiving components, including reservoirs, tubing, manifolds, or fluidic components of other systems. Additionally, the microfluidic channel structure  100  may have other geometries, including geometries having more than one channel junction. For example, the microfluidic channel structure can have 2, 3, 4, or 5 channel segments each carrying biological sample particles, assay reagents, and/or beads that meet at a channel junction. 
     Generally, the fluids used in generating the discrete droplets are directed to flow along one or more channels or reservoirs via one or more fluid flow units. A fluid flow unit can comprise compressors (e.g., providing positive pressure), pumps (e.g., providing negative pressure), actuators, and the like to control flow of the fluid. Fluid may also or otherwise be controlled via applied pressure differentials, centrifugal force, electro-kinetic pumping, vacuum, capillary or gravity flow, or the like. 
     One of ordinary skill will recognize that numerous different microfluidic channel designs are available that can be used with the methods and compositions of the present disclosure to provide discrete droplets containing a fixed biological sample particle, an un-fixing agent, and/or a bead with a barcode and/or other assay reagents. 
     The inclusion of a barcode in a discrete droplet along with the biological sample provides a unique identifier that allows data from the biological sample to be distinguished and individually analyzed. Barcodes can be delivered previous to, subsequent to, or concurrent with the biological sample in discrete droplet. For example, barcodes may be injected into droplets previous to, subsequent to, or concurrently with droplet generation. Barcodes useful in the methods and compositions of the present disclosure typically comprise a nucleic acid molecule (e.g., an oligonucleotide). The nucleic acid barcode molecules typically are delivered to a partition via a support, such as bead. In some cases, barcode nucleic acid molecules are initially associated with the bead upon generation of the discrete droplet, and then released from the bead upon application of a stimulus to droplet. Barcode carrying beads useful in the methods and compositions of the present disclosure are described in further detail elsewhere herein. 
     Methods and systems for partitioning barcode carrying beads into droplets are provided in U.S. Pat. Nos. 1,048,0029, 10,858,702, and 10,725,027, US. Patent Publication Nos. 2019/0367997 and 2019/0064173, and International Application Nos. PCT/US20/17785 and PCT/US20/020486, each of which is herein entirely incorporated by reference for all purposes. 
       FIG. 15  illustrates an example of a barcode carrying bead. A nucleic acid molecule  1502 , such as an oligonucleotide, can be coupled to a bead  1504  by a releasable linkage  1506 , such as, for example, a disulfide linker. The same bead  1504  may be coupled (e.g., via releasable linkage) to one or more other nucleic acid molecules  1518 ,  1520 . The nucleic acid molecule  1502  may be or comprise a barcode. As noted elsewhere herein, the structure of the barcode may comprise a number of sequence elements. The nucleic acid molecule  1502  may comprise a functional sequence  1508  that may be used in subsequent processing. For example, the functional sequence  1508  may include one or more of a sequencer specific flow cell attachment sequence (e.g., a P5 sequence for Illumine® sequencing systems) and a sequencing primer sequence (e.g., a R1 primer for Illumine® sequencing systems). The nucleic acid molecule  1502  may comprise a barcode sequence  1510  for use in barcoding the sample (e.g., DNA, RNA, protein, antibody, etc.). In some cases, the barcode sequence  1510  can be bead-specific such that the barcode sequence  1510  is common to all nucleic acid molecules (e.g., including nucleic acid molecule  1502 ) coupled to the same bead  1504 . Alternatively or in addition, the barcode sequence  1510  can be partition-specific such that the barcode sequence  1510  is common to all nucleic acid molecules coupled to one or more beads that are partitioned into the same partition. The nucleic acid molecule  1502  may comprise a specific priming sequence  1512 , such as an mRNA specific priming sequence (e.g., poly-T sequence), a targeted priming sequence, and/or a random priming sequence. The nucleic acid molecule  1502  may comprise an anchoring sequence  1514  to ensure that the specific priming sequence  1512  hybridizes at the sequence end (e.g., of the mRNA). For example, the anchoring sequence  1514  can include a random short sequence of nucleotides, such as a 1-mer, 2-mer, 3-mer or longer sequence, which can ensure that a poly-T segment is more likely to hybridize at the sequence end of the poly-A tail of the mRNA. 
     The nucleic acid molecule  1502  may comprise a unique molecular identifying sequence  1516  (e.g., unique molecular identifier (UMI)). In some cases, the unique molecular identifying sequence  1516  may comprise from about 5 to about 8 nucleotides. Alternatively, the unique molecular identifying sequence  1516  may compress less than about 5 or more than about 8 nucleotides. The unique molecular identifying sequence  1516  may be a unique sequence that varies across individual nucleic acid molecules (e.g.,  1502 ,  1518 ,  1520 , etc.) coupled to a single bead (e.g., bead  1504 ). In some cases, the unique molecular identifying sequence  1516  may be a random sequence (e.g., such as a random N-mer sequence). For example, the UMI may provide a unique identifier of the starting mRNA molecule that was captured, in order to allow quantitation of the number of original expressed RNA. As will be appreciated, although  FIG. 15  shows three nucleic acid molecules  1502 ,  1518 ,  1520  coupled to the surface of the bead  1504 , an individual bead may be coupled to any number of individual nucleic acid molecules, for example, from one to tens to hundreds of thousands or even millions of individual nucleic acid molecules. The respective barcodes for the individual nucleic acid molecules can comprise both common sequence segments or relatively common sequence segments (e.g.,  1508 ,  1510 ,  1512 , etc.) and variable or unique sequence segments (e.g.,  1516 ) between different individual nucleic acid molecules coupled to the same bead. 
     A biological particle (e.g., cell, fixed cell, un-fixed cell, DNA, RNA, etc.) can be co-partitioned along with a barcode bearing bead  1504 . The barcoded nucleic acid molecules  1502 ,  1518 ,  1520  can be released from the bead  1504  in the partition. By way of example, in the context of analyzing sample RNA, the poly-T segment (e.g.,  1512 ) of one of the released nucleic acid molecules (e.g.,  1502 ) can hybridize to the poly-A tail of a mRNA molecule. Reverse transcription may result in a cDNA transcript of the mRNA, but which transcript includes each of the sequence segments  1508 ,  1510 ,  1516  of the nucleic acid molecule  1502 . Because the nucleic acid molecule  1502  comprises an anchoring sequence  1514 , it will more likely hybridize to and prime reverse transcription at the sequence end of the poly-A tail of the mRNA. Within any given partition, all of the cDNA transcripts of the individual mRNA molecules may include a common barcode sequence segment  1510 . 
     However, the transcripts made from the different mRNA molecules within a given partition may vary at the unique molecular identifying sequence  1512  segment (e.g., UMI segment). Beneficially, even following any subsequent amplification of the contents of a given partition, the number of different UMIs can be indicative of the quantity of mRNA originating from a given partition, and thus from the biological particle (e.g., a cell, a fixed cell, an un-fixed cell, etc.). As noted above, the transcripts can be amplified, cleaned up and sequenced to identify the sequence of the cDNA transcript of the mRNA, as well as to sequence the barcode segment and the UMI segment. While a poly-T primer sequence is described, other targeted or random priming sequences may also be used in priming the reverse transcription reaction. Likewise, although described as releasing the barcoded oligonucleotides into the partition, in some cases, the nucleic acid molecules bound to the bead (e.g., gel bead) may be used to hybridize and capture the mRNA on the solid phase of the bead, for example, in order to facilitate the separation of the RNA from other cell contents. In such cases, further processing may be performed, in the partitions or outside the partitions (e.g., in bulk). For instance, the RNA molecules on the beads may be subjected to reverse transcription or other nucleic acid processing, additional adapter sequences may be added to the barcoded nucleic acid molecules, or other nucleic acid reactions (e.g., amplification, nucleic acid extension) may be performed. The beads or products thereof (e.g., barcoded nucleic acid molecules) may be collected from the partitions, and/or pooled together and subsequently subjected to clean up and further characterization (e.g., sequencing). The operations described herein may be performed at any useful or convenient step. For instance, the beads comprising nucleic acid barcode molecules may be introduced into a partition (e.g., well or droplet) prior to, during, or following introduction of a sample into the partition. The nucleic acid molecules of a sample may be subjected to barcoding, which may occur on the bead (in cases where the nucleic acid molecules remain coupled to the bead) or following release of the nucleic acid barcode molecules into the partition. In cases where the nucleic acid molecules from the sample remain attached to the bead, the beads from various partitions may be collected, pooled, and subjected to further processing (e.g., reverse transcription, adapter attachment, amplification, clean up, sequencing). In other instances, the processing may occur in the partition. For example, conditions sufficient for barcoding, adapter attachment, reverse transcription, or other nucleic acid processing operations may be provided in the partition and performed prior to clean up and sequencing. 
       FIG. 16  illustrates another example of a barcode carrying bead. A nucleic acid molecule  1605 , such as an oligonucleotide, can be coupled to a bead  1604  by a releasable linkage  1606 , such as, for example, a disulfide linker. The nucleic acid molecule  1605  may comprise a first capture sequence  1660 . The same bead  1604  may be coupled (e.g., via releasable linkage) to one or more other nucleic acid molecules  1603 ,  1607  comprising other capture sequences. The nucleic acid molecule  1605  may be or comprise a barcode. As noted elsewhere herein, the structure of the barcode may comprise a number of sequence elements, such as a functional sequence  1608  (e.g., flow cell attachment sequence, sequencing primer sequence, etc.), a barcode sequence  1610  (e.g., bead-specific sequence common to bead, partition-specific sequence common to partition, etc.), and a unique molecular identifier  1612  (e.g., unique sequence within different molecules attached to the bead), or partial sequences thereof. The capture sequence  1660  may be configured to attach to a corresponding capture sequence  1665 . In some instances, the corresponding capture sequence  1665  may be coupled to another molecule that may be an analyte or an intermediary carrier. For example, as illustrated in  FIG. 16 , the corresponding capture sequence  1665  is coupled to a guide RNA molecule  1662  comprising a target sequence  1664 , wherein the target sequence  1664  is configured to attach to the analyte. Another oligonucleotide molecule  1607  attached to the bead  1604  comprises a second capture sequence  1680  which is configured to attach to a second corresponding capture sequence  1685 . As illustrated in  FIG. 16 , the second corresponding capture sequence  1685  is coupled to an antibody  1682 . In some cases, the antibody  1682  may have binding specificity to an analyte (e.g., surface protein). Alternatively, the antibody  1682  may not have binding specificity. Another oligonucleotide molecule  1603  attached to the bead  1604  comprises a third capture sequence  1670  which is configured to attach to a second corresponding capture sequence  1675 . As illustrated in  FIG. 16 , the third corresponding capture sequence  1675  is coupled to a molecule  1672 . The molecule  1672  may or may not be configured to target an analyte. The other oligonucleotide molecules  1603 ,  1607  may comprise the other sequences (e.g., functional sequence, barcode sequence, UMI, etc.) described with respect to oligonucleotide molecule  1605 . While a single oligonucleotide molecule comprising each capture sequence is illustrated in  FIG. 16 , it will be appreciated that, for each capture sequence, the bead may comprise a set of one or more oligonucleotide molecules each comprising the capture sequence. For example, the bead may comprise any number of sets of one or more different capture sequences. Alternatively, or in addition, the bead  1604  may comprise other capture sequences. Alternatively, or in addition, the bead  1604  may comprise fewer types of capture sequences (e.g., two capture sequences). Alternatively or in addition, the bead  1604  may comprise oligonucleotide molecule(s) comprising a priming sequence, such as a specific priming sequence such as an mRNA specific priming sequence (e.g., poly-T sequence), a targeted priming sequence, and/or a random priming sequence, for example, to facilitate an assay for gene expression. 
       FIG. 2  shows an exemplary microfluidic channel structure  200  for generating discrete droplets comprising or encapsulating a barcode carrying bead  214  along with a biological sample particle  216 . The channel structure  200  includes channel segments  201 ,  202 ,  204 ,  206  and  208  in fluid communication at a channel junction  210 . In operation, the channel segment  201  transports an aqueous fluid  212  that can include a plurality of beads  214  (e.g., gel beads carrying barcode oligonucleotides) along the channel segment  201  into junction  210 . The plurality of beads  214  may be sourced from a suspension of beads. For example, the channel segment  201  can be connected to a reservoir comprising an aqueous suspension of beads  214 . The channel segment  202  transports the aqueous fluid  212  that includes a plurality of biological sample particles  216  along the channel segment  202  into junction  210 . The plurality of biological sample particles  216  may be sourced from a suspension of biological sample particles. For example, the channel segment  202  may be connected to a reservoir comprising an aqueous suspension of biological sample particles  216 . In some instances, the aqueous fluid  212  in either the first channel segment  201  or the second channel segment  202 , or in both segments, can include one or more reagents, as further described elsewhere herein. For example, in some embodiments of the present disclosure, where the biological sample particles are fixed biological sample particles, the aqueous fluid in the first and/or second channel segments that delivers the biological sample and beads, respectively, can include an un-fixing agent. The second fluid  218  that is immiscible with the aqueous fluid  212  is delivered to the junction  210  from each of channel segments  204  and  206 . Upon meeting of the aqueous fluid  212  from each of channel segments  201  and  202  and the second fluid  218  (e.g., a fluorinated oil) from each of channel segments  204  and  206  at the channel junction  210 , the aqueous fluid  212  is partitioned into discrete droplets  220  in the second fluid  218  and flow away from the junction  210  along channel segment  208 . The channel segment  208  can then deliver the discrete droplets encapsulating the biological sample particle and barcode carrying bead to an outlet reservoir fluidly coupled to the channel segment  208 , where they can be collected. 
     As an alternative, the channel segments  201  and  202  may meet at another junction upstream of the junction  210 . At such junction, beads and biological particles may form a mixture that is directed along another channel to the junction  210  to yield droplets  220 . The mixture may provide the beads and biological particles in an alternating fashion, such that, for example, a droplet comprises a single bead and a single biological particle. 
     Using such a channel system as exemplified in  FIG. 2 , discrete droplets  220  can be generated that comprise or encapsulate an individual biological particle of a biological sample, and one bead, wherein the bead can carry a barcode and/or another reagent. It is also contemplated, that in some instances, a discrete droplet may be generated using the channel system of  FIG. 2 , wherein droplet includes more than one individual biological sample particle or includes no biological sample. Similarly, in some embodiments, the discrete droplet may include more than one bead or no bead. A discrete droplet also may be completely unoccupied (e.g., no bead or biological sample). 
     In some embodiments, it is desired that the beads, biological sample particles, and generated discrete droplets flow along channels at substantially regular flow rates that generate a discrete droplet containing a single bead and a single biological sample particle. Regular flow rates and devices that may be used to provide such regular flow rates are known in the art, see e.g., U.S. Patent Publication No. 2015/0292988, which is hereby incorporated by reference herein in its entirety. In some embodiments, the flow rates are set to provide discrete droplets containing a single bead and a biological sample particle with a yield rate of greater than 5%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, or 95%. 
     D. Supports 
     Supports that can carry barcodes and/or other reagents are useful with the compositions and methods of the present disclosure and can include, without limitation, beads that are porous, non-porous, solid, semi-solid, semi-fluidic, fluidic, and/or a combination thereof. In some embodiments, the bead can be made of a material that is dissolvable, disruptable, and/or degradable, such as a gel bead comprising a hydrogel. Alternatively, in some embodiments, the bead is not degradable. 
     In some embodiments of the present disclosure, the bead provided in a discrete partition with a biological sample is a gel bead. In one embodiment, the bead provided in or encapsulated in a discrete droplet with a biological sample is a gel bead. Typically, the bead useful in the embodiments disclosed herein comprise a hydrogel. Such gel beads can be formed from molecular precursors, such as a polymeric or monomeric species, that undergo a reaction to form crosslinked gel polymer. Another semi-solid bead useful in the present disclosure is a liposomal bead. In some embodiments, beads used can be solid beads that comprise a metal including iron oxide, gold, and silver. In some cases, the bead may be a silica bead. In some cases, the bead can be rigid. In other cases, the bead may be flexible and/or compressible. Generally, the beads can be of any suitable shape. Examples of bead shapes include, but are not limited to, spherical, non-spherical, oval, oblong, amorphous, circular, cylindrical, and variations thereof. 
     The plurality beads used in the embodiments can be of uniform size or they can comprise a collection of heterogeneous sizes. In some cases, the diameter of a bead is at least about 1 micron (μm), 5 μm, 10 μm, 20 μm, 30 μm, 40 μm, 50 μm, 60 μm, 70 μm, 80 μm, 90 μm, 100 μm, 250 μm, 500 μm, 1000 μm (1 mm), or greater. In some cases, a bead may have a diameter of less than about 1 μm, 5 μm, 10 μm, 20 μm, 30 μm, 40 μm, 50 μm, 60 μm, 70 μm, 80 μm, 90 μm, 100 μm, 250 μm, 500 μm, 1 mm, or less. In some cases, a bead may have a diameter in the range of about 40-75 μm, 30-75 μm, 20-75 μm, 40-85 μm, 40-95 μm, 20-100 μm, 10-100 μm, 1-100 μm, 20-250 μm, or 20-500 μm. 
     In some embodiments, the beads used are a population or plurality of beads having a relatively monodisperse size distribution. Typically, where it is desirable to provide a consistent amount of a reagent within a partition (e.g., a well or a discrete droplet), the use of relatively consistent bead characteristics, such as size, provides overall consistency in the content of each partition. For example, the beads useful in the embodiments of the present disclosure can have size distributions that have a coefficient of variation in their cross-sectional dimensions of less than 50%, less than 40%, less than 30%, less than 20%, and in some cases less than 15%, less than 10%, less than 5%, or less. 
     The beads useful in the methods and compositions of the present disclosure can comprise a range of natural and/or synthetic materials. For example, a bead can comprise a natural polymer, a synthetic polymer or both natural and synthetic polymers. Examples of natural polymers include proteins and sugars such as deoxyribonucleic acid, rubber, cellulose, starch (e.g., amylose, amylopectin), proteins, enzymes, polysaccharides, silks, polyhydroxyalkanoates, chitosan, dextran, collagen, carrageenan, ispaghula, acacia, agar, gelatin, shellac, sterculia gum, xanthan gum, corn sugar gum, guar gum, gum karaya, agarose, alginic acid, alginate, or natural polymers thereof. Examples of synthetic polymers include acrylics, nylons, silicones, spandex, viscose rayon, polycarboxylic acids, polyvinyl acetate, polyacrylamide, polyacrylate, polyethylene glycol, polyurethanes, polylactic acid, silica, polystyrene, polyacrylonitrile, polybutadiene, polycarbonate, polyethylene, polyethylene terephthalate, poly(chlorotrifluoroethylene), poly(ethylene oxide), poly(ethylene terephthalate), polyethylene, polyisobutylene, poly(methyl methacrylate), poly(oxymethylene), polyformaldehyde, polypropylene, polystyrene, poly(tetrafluoroethylene), poly(vinyl acetate), poly(vinyl alcohol), poly(vinyl chloride), poly(vinylidene dichloride), poly(vinylidene difluoride), poly(vinyl fluoride) and/or combinations (e.g., co-polymers) thereof. Beads may also be formed from materials other than polymers, including lipids, micelles, ceramics, glass-ceramics, material composites, metals, other inorganic materials, and others. 
     Although  FIG. 1  and  FIG. 2  have been described in terms of providing substantially singly occupied discrete droplets, it is also contemplated in certain embodiments that it is desirable to provide multiply occupied discrete droplets, e.g., a single droplet that contains two, three, four or more cells from a biological sample, and/or multiple different beads, such as a bead carrying a barcode nucleic acid molecule and/or a bead carrying a reagent such as an un-fixing agent or assay reagent. Accordingly, as noted elsewhere herein, the flow characteristics of the biological particle and/or the beads can be controlled to provide for such multiply occupied droplets. In particular, the flow parameters of the liquids used in the channel structures may be controlled to provide a given droplet occupancy rate greater than about 50%, greater than about 75%, and in some cases greater than about 80%, 90%, 95%, or higher. 
     In some embodiments, the beads useful in the compositions and methods of the present disclosure are beads capable of delivering reagents (e.g., an un-fixing agent, and/or an assay reagent) into the discrete partition (e.g., a discrete droplet) containing the biological sample particle. In some embodiments, the different beads (e.g., containing different reagents) can be introduced from different sources into different inlets leading to a common droplet generation junction (e.g., junction  210 ). In such cases, the flow and frequency of the different beads into the channel or junction may be controlled to provide for a certain ratio of beads from each source, while ensuring a given pairing or combination of such beads into a partition with a given number of biological particles (e.g., one biological particle and one bead per partition). 
     The discrete droplets described herein generally comprise small volumes, for example, less than about 10 microliters (μL), 5 μL, 1 μL, 900 picoliters (μL), 800 μL, 700 μL, 600 μL, 500 μL, 400 μL, 300 μL, 200 μL, 100 μL, 50 μL, 20 μL, 10 μL, 1 μL, 500 nanoliters (nL), 100 nL, 50 nL, or less. In some embodiments, the discrete droplets generated that comprise or encapsulate a biological particle from a biological sample have overall volumes that are less than about 1000 μL, 900 μL, 800 μL, 700 μL, 600 μL, 500 μL, 400 μL, 300 μL, 200 μL, 100 μL, 50 μL, 20 μL, 10 μL, 1 μL, or less. It will be appreciated that the sample fluid volume, e.g., including co-partitioned biological particles and/or beads, within the droplets may be less than about 90% of the above described volumes, less than about 80%, less than about 70%, less than about 60%, less than about 50%, less than about 40%, less than about 30%, less than about 20%, or less than about 10% of the above described volumes. 
     The methods of generating discrete droplets useful with the compositions and methods of the present disclosure, result in the generation of a population or plurality of discrete droplets containing a biological sample particle (e.g., a fixed biological sample) and other reagents (e.g., an un-fixing agent). Generally, the methods are easily controlled to provide for any suitable number of droplets. For example, at least about 1,000 discrete droplets, at least about 5,000 discrete droplets, at least about 10,000 discrete droplets, at least about 50,000 discrete droplets, at least about 100,000 discrete droplets, at least about 500,000 discrete droplets, at least about 1,000,000 discrete droplets, at least about 5,000,000 discrete droplets, at least about 10,000,000 discrete droplets, or more discrete droplets can be generated or otherwise provided. Moreover, the plurality of discrete droplets may comprise both unoccupied and occupied droplets. 
     As described elsewhere herein, in some embodiments of the compositions and methods of the present disclosure, the generated discrete droplets comprising or encapsulating a biological sample particle, and optionally, one or more different beads, also contain other reagents. In some embodiments, the other reagents provided in or encapsulated in the droplet include lysis and/or un-fixing agents that act to release and/or un-fix the biomolecule contents of the biological sample particle within the droplet. In some embodiments, the lysis and/or un-fixing agents can be contacted with the biological sample suspension concurrently with, or immediately prior to, the introduction of the biological sample particles into the droplet generation junction of the microfluidic system (e.g., junction  210 ). In some embodiments, the agents are introduced through an additional channel or channels upstream of the channel junction. 
     In some embodiments, a biological sample particle can be co-partitioned along with the other reagents.  FIG. 3  shows an example of a microfluidic channel structure  300  for co-partitioning biological sample particles and other reagents, including lysis and/or un-fixing agents. The channel structure  300  can include channel segments  301 ,  302 ,  304 ,  306  and  308 . Channel segments  301  and  302  communicate at a first channel junction  309 . Channel segments  302 ,  304 ,  306 , and  308  communicate at a second channel junction  310 . In exemplary co-partitioning operation, the channel segment  301  may transport an aqueous fluid  312  that includes a plurality of biological sample particles  314  (e.g., a fixed biological sample) along the channel segment  301  into the second junction  310 . As an alternative or in addition to, channel segment  301  may transport beads (e.g., beads that carry barcodes). For example, the channel segment  301  may be connected to a reservoir comprising an aqueous suspension of biological sample particles  314 . Upstream of, and immediately prior to reaching, the second junction  310 , the channel segment  301  may meet the channel segment  302  at the first junction  309 . The channel segment  302  can transport a plurality of reagents  315  (e.g., lysis or un-fixing agents) in the aqueous fluid  312  along the channel segment  302  into the first junction  309 . For example, the channel segment  302  may be connected to a reservoir comprising the reagents  315 . After the first junction  309 , the aqueous fluid  312  in the channel segment  301  can carry both the biological sample particles  314  and the reagents  315  towards the second junction  310 . In some instances, the aqueous fluid  312  in the channel segment  301  can include one or more reagents, which can be the same or different reagents as the reagents  315 . A second fluid  316  that is immiscible with the aqueous fluid  312  (e.g., a fluorinated oil) can be delivered to the second junction  310  from each of channel segments  304  and  306 . Upon meeting of the aqueous fluid  312  from the channel segment  301  and the second fluid  316  from each of channel segments  304  and  306  at the second channel junction  310 , the aqueous fluid  312  is partitioned as discrete droplets  318  in the second fluid  316  and flow away from the second junction  310  along channel segment  308 . The channel segment  308  may deliver the discrete droplets  318  to an outlet reservoir fluidly coupled to the channel segment  308 , where they may be collected for further analysis. 
     Discrete droplets generated can include an individual biological sample particle  314  and/or one or more reagents  315 , depending on what reagents are included in channel segment  302 . In some instances, a discrete droplet generated may also include a barcode carrying bead (not shown), such as can be added via other channel structures described elsewhere herein. In some instances, a discrete droplet may be unoccupied (e.g., no reagents, no biological particles). Generally, the channel segments described herein may be coupled to any of a variety of different fluid sources or receiving components, including reservoirs, tubing, manifolds, or fluidic components of other systems. As will be appreciated, the microfluidic channel structure  300  may have other geometries. For example, a microfluidic channel structure can have more than two channel junctions. For example, a microfluidic channel structure can have 2, 3, 4, 5 channel segments or more each carrying the same or different types of beads, reagents, and/or biological sample particles that meet at a channel junction. Fluid flow in each channel segment may be controlled to control the partitioning of the different elements into droplets. Fluid may be directed flow along one or more channels or reservoirs via one or more fluid flow units. A fluid flow unit can comprise compressors (e.g., providing positive pressure), pumps (e.g., providing negative pressure), actuators, and the like to control flow of the fluid. Fluid may also or otherwise be controlled via applied pressure differentials, centrifugal force, electro-kinetic pumping, vacuum, capillary or gravity flow, or the like. 
       FIG. 4  shows an example of a microfluidic channel structure for the controlled partitioning of beads into discrete droplets. A channel structure  400  can include a channel segment  402  communicating at a channel junction  406  (or intersection) with a reservoir  404 . The reservoir  404  can be a chamber. Any reference to “reservoir,” as used herein, can also refer to a “chamber.” In operation, an aqueous fluid  408  that includes suspended beads  412  may be transported along the channel segment  402  into the junction  406  to meet a second fluid  410  that is immiscible with the aqueous fluid  408  in the reservoir  404  to create droplets  416 ,  418  of the aqueous fluid  408  flowing into the reservoir  404 . At the junction  406  where the aqueous fluid  408  and the second fluid  410  meet, droplets can form based on factors such as the hydrodynamic forces at the junction  406 , flow rates of the two fluids  408 ,  410 , fluid properties, and certain geometric parameters (e.g., w, h 0 , α, etc.) of the channel structure  400 . A plurality of droplets can be collected in the reservoir  404  by continuously injecting the aqueous fluid  408  from the channel segment  402  through the junction  406 . 
       FIG. 5  shows an example of a microfluidic channel structure for increased droplet generation throughput. A microfluidic channel structure  500  can comprise a plurality of channel segments  502  and a reservoir  504 . Each of the plurality of channel segments  502  may be in fluid communication with the reservoir  504 . The channel structure  500  can comprise a plurality of channel junctions  506  between the plurality of channel segments  502  and the reservoir  504 . Each channel junction can be a point of droplet generation. The channel segment  402  from the channel structure  400  in  FIG. 4  and any description to the components thereof may correspond to a given channel segment of the plurality of channel segments  502  in channel structure  500  and any description to the corresponding components thereof. The reservoir  404  from the channel structure  400  and any description to the components thereof may correspond to the reservoir  504  from the channel structure  500  and any description to the corresponding components thereof. 
       FIG. 6  shows another example of a microfluidic channel structure for increased droplet generation throughput. A microfluidic channel structure  600  can comprise a plurality of channel segments  602  arranged generally circularly around the perimeter of a reservoir  604 . Each of the plurality of channel segments  602  may be in fluid communication with the reservoir  604 . The channel structure  600  can comprise a plurality of channel junctions  606  between the plurality of channel segments  602  and the reservoir  604 . Each channel junction can be a point of droplet generation. The channel segment  402  from the channel structure  400  in  FIG. 4  and any description to the components thereof may correspond to a given channel segment of the plurality of channel segments  602  in channel structure  600  and any description to the corresponding components thereof. The reservoir  404  from the channel structure  400  and any description to the components thereof may correspond to the reservoir  604  from the channel structure  600  and any description to the corresponding components thereof. Additional aspects of the microfluidic structures depicted in  FIGS. 4-6 , including systems and methods implementing the same, are provided in US Published Patent Application No 20190323088, which is incorporated herein by reference in its entirety. 
     Once the lysis and/or un-fixing agents are co-partitioned in a partition (e.g., a well or a droplet) with a fixed biological sample particle, these reagents can facilitate the release and un-fixing of the biomolecular contents of the biological sample particle within the partition. As described elsewhere herein, the un-fixed biomolecular contents released in a partition remain discrete from the contents of other partitions, thereby allowing for detection and quantitation of the biomolecular analytes of interest present in that distinct biological sample. 
     Examples of lysis agents useful in the compositions and methods of the present disclosure include bioactive reagents, such as lysis enzymes that are used for lysis of different cell types, e.g., gram positive or negative bacteria, plants, yeast, mammalian, etc., such as lysozymes, achromopeptidase, lysostaphin, labiase, kitalase, lyticase, and a variety of other lysis enzymes available from, e.g., Sigma-Aldrich, Inc. (St Louis, Mo.), as well as other commercially available lysis enzymes. Other lysis agents may additionally or alternatively be co-partitioned with the biological particles to cause the release of the biological samples&#39; contents into the partition (e.g., a well or a droplet). For example, in some cases, surfactant-based lysis solutions may be used to lyse cells, although these may be less desirable for emulsion based systems where the surfactants can interfere with stable emulsions. In some embodiment, the lysis solutions can include non-ionic surfactants such as, for example, TritonX-100 and Tween 20. In some cases, lysis solutions may include ionic surfactants such as, for example, sarcosyl and sodium dodecyl sulfate (SDS). Electroporation, thermal, acoustic or mechanical cellular disruption may also be used in certain cases, e.g., non-emulsion based partitioning such as the provision or encapsulation of biological particles that may be in addition to or in place of droplet partitioning, where any pore size of the encapsulating material is sufficiently small to retain nucleic acid fragments of a given size, following cellular disruption. 
     In addition to the lysis and/or un-fixing agents co-partitioned into discrete partitions (e.g., wells or droplets) with the biological sample particles, it is further contemplated that other assay reagents can also be co-partitioned in the partition. For example, DNase and RNase inactivating agents or inhibitors, chelating agents, such as EDTA, proteases, such as subtilisin A, proteinase K,  Serratia  peptidase peptidase derived from  Serratia  sp.), ArcticZymes Proteinase, and other reagents employed in removing or otherwise reducing negative activity or impact of different cell lysate components on subsequent processing of nucleic acids. 
     The inclusion of a composition of a protease in combination with an un-fixing agent in a partition (e.g., a well or a discrete droplet), greatly facilitates the process of un-fixing the biological sample necessary for subsequent assay of the sample. Accordingly, in at least one embodiment, a protease is co-partitioned into a discrete partition with an un-fixing agent and a fixed biological sample. A suitable class of proteases for use in the methods of the present disclosure are serine proteases (E.C. 3.4.21), which can include chymotrypsin-like, trypsin-like, thrombin-like, elastase-like, and subtilisin-like proteases. A wide range of different serine proteases are well-characterized and commercially available. Among the serine proteases that may be useful in the methods of the present selected are: alcalase, alkaline proteinase, ArcticZymes Proteinase, bacillopeptidase A, bacillopeptidase B, bioprase, colistinase, esperase, genenase, kazusase, maxatase,  Serratia  peptidase, proteinase K, protease S, savinase, subtilisin A, subtilisin B, subtilisin BL, subtilisin E, subtilisin J, subtilisin S, subtilisin S41, Thermolabile Proteinase K (New England Biolabs), thermoase, and trypsin. 
     In some embodiments, the biological particles from a biological sample are provided in or encapsulated in discrete partitions (e.g., wells or droplets) with other reagents are exposed to an appropriate stimulus to release the biomolecular contents of the sample particles and/or the contents of a co-partitioned bead. For example, in some embodiments, a chemical stimulus may be co-partitioned in the partition along with a biological sample particle and a bead (e.g., a gel bead) to allow for the degradation of the bead and release of its contents into the partition. In some embodiments, a discrete partition can be generated with a fixed biological sample particle and an un-fixing agent, wherein the un-fixing agent is contained in a bead that can be degraded by heat stimulus. In such an embodiment, the partition is exposed to heat stimulus thereby degrading the bead and releasing the un-fixing agent. In another embodiment, it is contemplated that a partition comprising a fixed biological sample particle (e.g., a fixed cell) and two different beads (e.g., one bead carrying an un-fixing agent, and one bead carrying assay reagents), wherein the contents of the two different beads are released by non-overlapping stimuli (e.g., a chemical stimulus and a heat stimulus). In one embodiment, the partition is a droplet comprising or encapsulating a fixed biological sample particle, and two different beads (e.g., one bead carrying an un-fixing agent, and one bead carrying assay reagents), wherein the contents of the two different beads are released by non-overlapping stimuli (e.g., a chemical stimulus and a heat stimulus). Such an embodiment can allow the release of the different reagents into the same discrete partition (e.g., a well or a droplet) at different times. For example, a first bead, triggered by heat stimulus, releases an un-fixing agent into the partition, and then after a set time, a second bead, triggered by a chemical stimulus, releases assay reagents that detect analytes of the un-fixed biological sample particle. 
     Additional assay reagents may also be co-partitioned into discrete partitions (e.g., wells or droplets) with the biological samples, such as endonucleases to fragment a biological sample&#39;s DNA, DNA polymerase enzymes and dNTPs used to amplify the biological sample&#39;s nucleic acid fragments and to attach the barcode molecular tags to the amplified fragments. Other enzymes may be co-partitioned, including without limitation, polymerase, transposase, ligase, proteinase K, DNase, subtilisin A,  Serratia  peptidase, ArcticZymes Proteinase, Thermolabile Proteinase K (New England Biolabs), etc. Additional assay reagents may also include reverse transcriptase (RT) enzymes, including enzymes with terminal transferase activity, primers and oligonucleotides, and switch oligonucleotides (also referred to herein as “switch oligos” or “template switching oligonucleotides”) which can be used for template switching. 
     In some embodiments, template switching can be used to increase the length of cDNA generated in an assay. In some embodiments, template switching can be used to append a predefined nucleic acid sequence to the cDNA. In an example of template switching, cDNA can be generated from reverse transcription of a template, e.g., cellular mRNA, where a reverse transcriptase (RT) with terminal transferase activity can add additional nucleotides, e.g., polyC, to the cDNA in a template independent manner. 
     Once the contents of a biological sample cell are released into a discrete partition (e.g., a well or a droplet), the biomolecular components (e.g., macromolecular constituents of biological samples, such as RNA, DNA, or proteins) contained therein may be further processed within the partition. In accordance with the methods and systems described herein, the biomolecular contents of individual biological samples can be provided with unique barcode identifiers, and upon characterization of the biomolecular components (e.g., in a sequencing assay) they may be attributed as having been derived from the same biological sample. The ability to attribute characteristics to individual biological samples or groups of biological samples is provided by the assignment of a nucleic acid barcode sequence specifically to an individual biological sample or groups of biological samples. 
     In some aspects, the unique identifier barcodes are provided in the form of nucleic acid molecules (e.g., oligonucleotides) that comprise sequences that may be attached to or otherwise associated with the nucleic acid contents of individual biological sample, or to other components of the biological sample, and particularly to fragments of those nucleic acids. In some embodiments, only one nucleic acid barcode sequence is associated with a given discrete droplet, although in some cases, two or more different barcode sequences may be present. The nucleic acid barcode sequences can include from about 6 to about 20 or more nucleotides within the sequence of the nucleic acid molecules (e.g., oligonucleotides). In some cases, the length of a barcode sequence may be about 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20 nucleotides or longer. In some cases, the length of a barcode sequence may be at least about 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20 nucleotides or longer. In some cases, the length of a barcode sequence may be at most about 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20 nucleotides or shorter. These nucleotides may be completely contiguous, i.e., in a single stretch of adjacent nucleotides, or they may be separated into two or more separate subsequences that are separated by 1 or more nucleotides. In some cases, separated barcode subsequences can be from about 4 to about 16 nucleotides in length. In some cases, the barcode subsequence may be about 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16 nucleotides or longer. In some cases, the barcode subsequence may be at least about 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16 nucleotides or longer. In some cases, the barcode subsequence may be at most about 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16 nucleotides or shorter. 
     In some embodiments, the nucleic acid barcode molecules can also comprise other functional sequences useful in the processing of the nucleic acids from the biological sample in the partition. These functional sequences can include, e.g., targeted or random/universal amplification primer sequences for amplifying the nucleic acid molecules from the individual biological samples within the partitions while attaching the associated barcode sequences, sequencing primers or primer recognition sites, hybridization or probing sequences, e.g., for identification of presence of the sequences or for pulling down barcoded nucleic acid molecules, or any of a number of other potential functional sequences. 
     In some embodiments, large numbers of nucleic acid barcode molecules (e.g., oligonucleotides) are releasably attached to beads, wherein all of the nucleic acid molecules attached to a particular bead will include the same nucleic acid barcode sequence, but where a large number of diverse barcode sequences are represented across the population of beads used. In some embodiments, gel beads (e.g., comprising polyacrylamide polymer matrices), are used as a solid support and delivery vehicle for the nucleic acid molecules into the partitions, as they are capable of carrying large numbers of nucleic acid molecules, and may be configured to release those nucleic acid molecules upon exposure to a particular stimulus, as described elsewhere herein. In some cases, the population of beads provides a diverse barcode sequence library that includes at least about 1,000 different barcode sequences, at least about 5,000 different barcode sequences, at least about 10,000 different barcode sequences, at least about 50,000 different barcode sequences, at least about 100,000 different barcode sequences, at least about 1,000,000 different barcode sequences, at least about 5,000,000 different barcode sequences, or at least about 10,000,000 different barcode sequences, or more. 
     The nucleic acid barcode molecules can be released from the beads upon the application of a particular stimulus to the beads. In some cases, the stimulus may be a photo-stimulus, e.g., through cleavage of a photo-labile linkage that releases the nucleic acid molecules. In other cases, a thermal stimulus may be used, where elevation of the temperature of the beads environment will result in cleavage of a linkage or other release of the nucleic acid molecules form the beads. In still other cases, a chemical stimulus can be used that cleaves a linkage of the nucleic acid molecules to the beads, or otherwise results in release of the nucleic acid molecules from the beads. In one case, such compositions include the polyacrylamide matrices described above for provision or encapsulation of biological samples and may be degraded for release of the attached nucleic acid molecules through exposure to a reducing agent, such as DTT. 
     E. Use of Fixed Biological Samples and Un-Fixing Agents in Partition-Based Assays 
     As disclosed elsewhere herein, the compositions and methods of the present disclosure allow a fixed, stabilized, biological sample (e.g., fixed cell(s) such as formaldehyde-fixed biopsy cells) to be provided in a discrete partition (e.g., a well or a droplet), optionally as a single fixed cell, together with an un-fixing agent that is capable of reversing the fixation and thereby allowing the cellular analytes of the sample to be assayed as if they were obtained from a fresh sample. In one embodiment, the fixed, stabilized, biological sample (e.g., fixed cell(s) such as formaldehyde-fixed biopsy cells) is provided or encapsulated in a discrete droplet (optionally, as a single fixed cell) together with an un-fixing agent, that is capable of reversing the fixation and thereby allowing the cellular analytes of the sample to be assayed as if they were obtained from a fresh sample. These methods allow for a fresh biological sample to be immediately fixed e.g., with formaldehyde, and then stored for a period of time before it is provided in a partition (e.g., provided in a well, or in a droplet, or encapsulated in a droplet) with an un-fixing agent, and typically with other materials such as unique nucleic acid barcode molecule and assay reagents. Accordingly, it is contemplated that the methods of the present disclosure can be carried out wherein the amount of time prior to generating the discrete partition when the biological sample is fixed is at least 1 hour, at least 2 hours, at least 6 hours, at least 12 hours, at least 24 hours, at least 1 week, at least 1 month, at least 6 months, or longer. 
     The present disclosure also provides an assay method that comprises the steps of: (a) providing a discrete partition (e.g., a well or a droplet) comprising a fixed biological sample, an un-fixing agent, and assay reagents; and (b) detecting analytes from the reaction of the assay reagents and the un-fixed biological sample. In one embodiment, the assay method comprises the steps of: (a) generating a discrete droplet comprising or encapsulating a fixed biological sample, an un-fixing agent, and assay reagents; and (b) detecting analytes from the reaction of the assay reagents and the un-fixed biological sample. Optionally, the steps of the method can further comprise fixing the biological sample prior to generating the discrete partition (e.g., discrete well or discrete droplet). 
     A wide range of droplet-based assays and systems are known in the art. Assays and systems that are suitable for use with the present invention include, without limitation, those described in U.S. Pat. Nos. 9,694,361, 10,357,771, 10,273,541, and 10,011,872, as well as US Published Patent Application Nos. 20180105808, 20190367982, and 20190338353, each of which is incorporated herein by reference in its entirety. It is contemplated that any assay that can be carried out using a fresh biological sample, such as a single cell provided or encapsulated in a droplet with a bead carrying a barcode, can also be carried out using a fixed biological sample, the unfixing agents as disclosed herein, and the methods of the present disclosure. That is, the in any droplet-based assay the fresh biological sample can be fixed prior to running the assay protocol, and the fixed biological sample used. In such an assay the protocol comprises providing or encapsulating the fixed biological together with an un-fixing agent and assay reagents in a discrete droplet. 
     Exemplary assays include single-cell transcription profiling, single-cell sequence analysis, immune profiling of individual T and B cells, single-cell chromatin accessibility analysis (e.g., ATAC seq analysis). These exemplary assays can be carried out using commercially available systems for partitioning or encapsulating biological samples, gel beads, barcodes, and/or other compounds/materials in droplets, such as the Chromium System (10× Genomics, Inc., Pleasanton, Calif., USA). 
     In some embodiments of the assay methods, the discrete partition (e.g., a well or a droplet) further comprises one or more beads. In some embodiments, the bead(s) can contain the assay reagents and/or the un-fixing agent. In some embodiments, a barcode is carried by or contained in a bead. Compositions, methods and systems for sample preparation, amplification, and sequencing of biomolecules from single cells provided or encapsulated with barcodes in droplets are provided in e.g., US Pat. Publication No. 20180216162A1, which is hereby incorporated by reference herein. 
     Assay reagents can include those used to perform one or more additional chemical or biochemical operations on a biological sample provided in a partition (e.g., provided in a well or in a droplet, or encapsulated in a droplet). Accordingly, assay reagents useful in the assay method include any reagents useful in performing a reaction such as nucleic acid modification (e.g., ligation, digestion, methylation, random mutagenesis, bisulfite conversion, uracil hydrolysis, nucleic acid repair, capping, or decapping), nucleic acid amplification (e.g., isothermal amplification or PCR), nucleic acid insertion or cleavage (e.g., via CRISPR/Cas9-mediated or transposon-mediated insertion or cleavage), and/or reverse transcription. Additionally, useful assay reagents can include those that allow the preparation of a target sequence or sequencing reads that are specific to the macromolecular constituents of interest at a higher rate than to non-target sequence specific reads. 
     In addition, the present invention provides compositions and systems related to the analysis of fixed biological samples. In one embodiment, the present invention provides a composition comprising a plurality of partitions, wherein a subset of said plurality of partitions comprises fixed cells and an un-fixing agent. In one other embodiment, the subset of partitions further comprises a protease. In another embodiment, a partition of the plurality of partitions comprises a fixed cell and an un-fixing agent. In certain embodiments, the fixed cell is a single fixed cell. In other embodiments the present invention provides a composition comprising a partition, wherein the partition comprises a fixed cell and an un-fixing agent, as described herein. The partition may be a droplet or a well. In another embodiment, the partition further comprises a protease. The partition or partitions described herein may further comprise one or more of the following: a reverse transcriptase (RT), a bead, and reagents for a nucleic acid extension reaction. In an additional embodiment, the compositions of the present invention have or are provided at a temperature other than ambient temperature or non-ambient temperature. In one embodiment, the temperature is below ambient temperature or above ambient temperature. As described elsewhere herein, partitioning approaches may generate a population or plurality of partitions. In such cases, any suitable number of partitions can be generated or otherwise provided. For example, at least about 1,000 partitions, at least about 5,000 partitions, at least about 10,000 partitions, at least about 50,000 partitions, at least about 100,000 partitions, at least about 500,000 partitions, at least about 1,000,000 partitions, at least about 5,000,000 partitions at least about 10,000,000 partitions, at least about 50,000,000 partitions, at least about 100,000,000 partitions, at least about 500,000,000 partitions, at least about 1,000,000,000 partitions, or more partitions can be generated or otherwise provided. Moreover, the plurality of partitions may comprise both unoccupied partitions (e.g., empty partitions) and occupied partitions. For example, an occupied partition according the present invention comprises a fixed cell and an un-fixing agent. 
     In another aspect, the present invention concerns methods and compositions for the partitioning of a plurality of fixed cells into individual partitions. In some cases, about 10, about 20, about 30, about 40, about 50, about 60, about 70, about 80, about 90, about 100, about 200, about 300, about 400, about 500, about 600, about 700, about 800, about 900, about 1000, about 2000, about 3000, about 4000, about 5000, about 6000, about 7000, about 8000, about 9000, about 10,000, about 15,000, about 20,000, about 25,000, about 30,000, about 35,000, about 40,000, about 50,000, about 60,000, about 70,000, about 80,000, about 90,000 or about 100,000 fixed cells may be partitioned into individual partitions. In some instances, the method further comprises partitioning about 50 to about 20,000 fixed cells with each of a plurality of supports comprising the adaptor comprising the barcode sequence, wherein the barcode sequence is unique among each of the plurality of supports. 
       FIG. 17  schematically illustrates an example of a microwell array. The array can be contained within a substrate  1700 . The substrate  1700  comprises a plurality of wells  1702 . The wells  1702  may be of any size or shape, and the spacing between the wells, the number of wells per substrate, as well as the density of the wells on the substrate  1700  can be modified, depending on the particular application. In one such example application, a sample molecule  1706 , which may comprise a cell (e.g., a fixed cell or an un-fixed cell) or cellular components (e.g., nucleic acid molecules) is co-partitioned with a bead  1704 , which may comprise a nucleic acid barcode molecule coupled thereto. The wells  1702  may be loaded using gravity or other loading technique (e.g., centrifugation, liquid handler, acoustic loading, optoelectronic, etc.). In some instances, at least one of the wells  1702  contains a single sample molecule  1706  (e.g., cell) and a single bead  1704 . 
     Reagents may be loaded into a well either sequentially or concurrently. In some cases, reagents are introduced to the device either before or after a particular operation. In some cases, reagents (which may be provided, in certain instances, in droplets or beads) are introduced sequentially such that different reactions or operations occur at different steps. The reagents (or droplets or beads) may also be loaded at operations interspersed with a reaction or operation step. For example, droplets or beads comprising reagents for fragmenting polynucleotides (e.g., restriction enzymes) and/or other enzymes (e.g., transposases, ligases, polymerases, etc.) may be loaded into the well or plurality of wells, followed by loading of droplets or beads comprising reagents for attaching nucleic acid barcode molecules to a sample nucleic acid molecule. Reagents may be provided concurrently or sequentially with a sample, such as a cell (e.g., a fixed cell or an un-fixed cell) or cellular components (e.g., organelles, proteins, nucleic acid molecules, carbohydrates, lipids, etc.). Accordingly, use of wells may be useful in performing multi-step operations or reactions. 
     As described elsewhere herein, the nucleic acid barcode molecules and other reagents may be contained within a bead or droplet. These beads or droplets may be loaded into a partition (e.g., a microwell) before, after, or concurrently with the loading of a cell (e.g., a fixed cell or an un-fixed cell), such that each cell is contacted with a different bead or droplet. This technique may be used to attach a unique nucleic acid barcode molecule to nucleic acid molecules obtained from each cell (e.g., a fixed cell or an un-fixed cell). Alternatively or in addition to, the sample nucleic acid molecules may be attached to a support. For instance, the partition (e.g., microwell) may comprise a bead which has coupled thereto a plurality of nucleic acid barcode molecules. The sample nucleic acid molecules, or derivatives thereof, may couple or attach to the nucleic acid barcode molecules on the support. The resulting barcoded nucleic acid molecules may then be removed from the partition, and in some instances, pooled and sequenced. In such cases, the nucleic acid barcode sequences may be used to trace the origin of the sample nucleic acid molecule. For example, polynucleotides with identical barcodes may be determined to originate from the same cell or partition, while polynucleotides with different barcodes may be determined to originate from different cells or partitions. 
     The samples or reagents may be loaded in the wells or microwells using a variety of approaches. The samples (e.g., a cell or cellular component) or reagents (as described herein) may be loaded into the well or microwell using an external force, e.g., gravitational force, electrical force, magnetic force, or using mechanisms to drive the sample or reagents into the well, e.g., via pressure-driven flow, centrifugation, optoelectronics, acoustic loading, electrokinetic pumping, vacuum, capillary flow, etc. In certain cases, a fluid handling system may be used to load the samples or reagents into the well. The loading of the samples or reagents may follow a Poissonian distribution or a non-Poissonian distribution, e.g., super Poisson or sub-Poisson. The geometry, spacing between wells, density, and size of the microwells may be modified to accommodate a useful sample or reagent distribution; for instance, the size and spacing of the microwells may be adjusted such that the sample or reagents may be distributed in a super-Poissonian fashion. 
     In one particular non-limiting example, the microwell array or plate comprises pairs of microwells, in which each pair of microwells is configured to hold a droplet (e.g., comprising a single cell, e.g., a single fixed cell or a single un-fixed cell) and a single bead (such as those described herein, which may, in some instances, also be provided or encapsulated in a droplet). The droplet and the bead (or droplet containing the bead) may be loaded simultaneously or sequentially, and the droplet and the bead may be merged, e.g., upon contact of the droplet and the bead, or upon application of a stimulus (e.g., external force, agitation, heat, light, magnetic or electric force, etc.). In some cases, the loading of the droplet and the bead is super-Poissonian. In other examples of pairs of microwells, the wells are configured to hold two droplets comprising different reagents and/or samples, which are merged upon contact or upon application of a stimulus. In such instances, the droplet of one microwell of the pair can comprise reagents that may react with an agent in the droplet of the other microwell of the pair. For instance, one droplet can comprise reagents that are configured to release the nucleic acid barcode molecules of a bead contained in another droplet, located in the adjacent microwell. Upon merging of the droplets, the nucleic acid barcode molecules may be released from the bead into the partition (e.g., the microwell or microwell pair that are in contact), and further processing may be performed (e.g., barcoding, nucleic acid reactions, etc.). In cases where cells, e.g., fixed cells or un-fixed cells are loaded in the microwells, one of the droplets may comprise reagents for further processing, e.g., lysis reagents for lysing the cell, upon droplet merging. 
     A droplet may be partitioned into a well. The droplets may be selected or subjected to pre-processing prior to loading into a well. For instance, the droplets may comprise cells, e.g., fixed cells or un-fixed cells, and only certain droplets, such as those containing a single cell (or at least one cell), may be selected for use in loading of the wells. Such a pre-selection process may be useful in efficient loading of single cells, such as to obtain a non-Poissonian distribution, or to pre-filter cells for a selected characteristic prior to further partitioning in the wells. Additionally, the technique may be useful in obtaining or preventing cell doublet or multiplet formation prior to or during loading of the microwell. 
     In some instances, the wells can comprise nucleic acid barcode molecules attached thereto. The nucleic acid barcode molecules may be attached to a surface of the well (e.g., a wall of the well). The nucleic acid barcode molecule (e.g., a partition barcode sequence) of one well may differ from the nucleic acid barcode molecule of another well, which can permit identification of the contents contained with a single partition or well. In some cases, the nucleic acid barcode molecule can comprise a spatial barcode sequence that can identify a spatial coordinate of a well, such as within the well array or well plate. In some cases, the nucleic acid barcode molecule can comprise a unique molecular identifier for individual molecule identification. In some instances, the nucleic acid barcode molecules may be configured to attach to or capture a nucleic acid molecule within a sample or cell (e.g., a fixed cell or an un-fixed cell) distributed in the well. For example, the nucleic acid barcode molecules may comprise a capture sequence that may be used to capture or hybridize to a nucleic acid molecule (e.g., RNA, DNA) within the sample. In some instances, the nucleic acid barcode molecules may be releasable from the microwell. For instance, the nucleic acid barcode molecules may comprise a chemical cross-linker which may be cleaved upon application of a stimulus (e.g., photo-, magnetic, chemical, biological, stimulus). The released nucleic acid barcode molecules, which may be hybridized or configured to hybridize to a sample nucleic acid molecule, may be collected and pooled for further processing, which can include nucleic acid processing (e.g., amplification, extension, reverse transcription, etc.) and/or characterization (e.g., sequencing). In such cases, the unique partition barcode sequences may be used to identify the cell or partition from which a nucleic acid molecule originated. 
     Characterization of samples within a well may be performed. Such characterization can include, in non-limiting examples, imaging of the sample (e.g., cell or cellular components) or derivatives thereof. Characterization techniques such as microscopy or imaging may be useful in measuring sample profiles in fixed spatial locations. For instance, when cells (e.g., fixed cells or un-fixed cells) are partitioned, optionally with beads, imaging of each microwell and the contents contained therein may provide useful information on cell doublet formation (e.g., frequency, spatial locations, etc.), cell-bead pair efficiency, cell viability, cell size, cell morphology, expression level of a biomarker (e.g., a surface marker, a fluorescently labeled molecule therein, etc.), cell or bead loading rate, number of cell-bead pairs, cell-cell interactions (when two or more cells are co-partitioned). Alternatively or in addition to, imaging may be used to characterize a quantity of amplification products in the well. 
     In operation, a well may be loaded with a sample and reagents, simultaneously or sequentially. When cells (e.g., fixed cells or un-fixed cells) are loaded, the well may be subjected to washing, e.g., to remove excess cells from the well, microwell array, or plate. Similarly, washing may be performed to remove excess beads or other reagents from the well, microwell array, or plate. In addition, the cells may be lysed in the individual partitions to release the intracellular components or cellular analytes. Alternatively, the cells may be fixed or permeabilized in the individual partitions. The intracellular components or cellular analytes may couple to a support, e.g., on a surface of the microwell, on a solid support (e.g., bead), or they may be collected for further downstream processing. For instance, after cell lysis, the intracellular components or cellular analytes may be transferred to individual droplets or other partitions for barcoding. Alternatively, or in addition to, the intracellular components or cellular analytes (e.g., nucleic acid molecules) may couple to a bead comprising a nucleic acid barcode molecule; subsequently, the bead may be collected and further processed, e.g., subjected to nucleic acid reaction such as reverse transcription, amplification, or extension, and the nucleic acid molecules thereon may be further characterized, e.g., via sequencing. Alternatively, or in addition to, the intracellular components or cellular analytes may be barcoded in the well (e.g., using a bead comprising nucleic acid barcode molecules that are releasable or on a surface of the microwell comprising nucleic acid barcode molecules). The barcoded nucleic acid molecules or analytes may be further processed in the well, or the barcoded nucleic acid molecules or analytes may be collected from the individual partitions and subjected to further processing outside the partition. Further processing can include nucleic acid processing (e.g., performing an amplification, extension) or characterization (e.g., fluorescence monitoring of amplified molecules, sequencing). At any convenient or useful step, the well (or microwell array or plate) may be sealed (e.g., using an oil, membrane, wax, etc.), which enables storage of the assay or selective introduction of additional reagents. 
       FIG. 18  schematically shows an example workflow for processing nucleic acid molecules within a sample. A substrate  1800  comprising a plurality of microwells  1802  may be provided. A sample  1806  which may comprise a cell (e.g., a fixed cell or an un-fixed cell), cellular components or analytes (e.g., proteins and/or nucleic acid molecules) can be co-partitioned, in a plurality of microwells  1802 , with a plurality of beads  1804  comprising nucleic acid barcode molecules. During process  1810 , the sample  1806  may be processed within the partition. For instance, the cell may be subjected to conditions sufficient to lyse the cells (e.g., fixed cells or un-fixed cells) and release the analytes contained therein. In process  1820 , the bead  1804  may be further processed. By way of example, processes  1820   a  and  1820   b  schematically illustrate different workflows, depending on the properties of the bead  1804 . 
     In  1820   a , the bead comprises nucleic acid barcode molecules that are attached thereto, and sample nucleic acid molecules (e.g., RNA, DNA) may attach, e.g., via hybridization of ligation, to the nucleic acid barcode molecules. Such attachment may occur on the bead. In process  1830 , the beads  1804  from multiple wells  1802  may be collected and pooled. Further processing may be performed in process  1840 . For example, one or more nucleic acid reactions may be performed, such as reverse transcription, nucleic acid extension, amplification, ligation, transposition, etc. In some instances, adapter sequences are ligated to the nucleic acid molecules, or derivatives thereof, as described elsewhere herein. For instance, sequencing primer sequences may be appended to each end of the nucleic acid molecule. In process  1850 , further characterization, such as sequencing may be performed to generate sequencing reads. The sequencing reads may yield information on individual cells or populations of cells (e.g., fixed cells or un-fixed cells), which may be represented visually or graphically, e.g., in a plot  1855 . 
     In  1820   b , the bead comprises nucleic acid barcode molecules that are releasably attached thereto, as described below. The bead may degrade or otherwise release the nucleic acid barcode molecules into the well  1802 ; the nucleic acid barcode molecules may then be used to barcode nucleic acid molecules within the well  1802 . Further processing may be performed either inside the partition or outside the partition. For example, one or more nucleic acid reactions may be performed, such as reverse transcription, nucleic acid extension, amplification, ligation, transposition, etc. In some instances, adapter sequences are ligated to the nucleic acid molecules, or derivatives thereof, as described elsewhere herein. For instance, sequencing primer sequences may be appended to each end of the nucleic acid molecule. In process  1850 , further characterization, such as sequencing may be performed to generate sequencing reads. The sequencing reads may yield information on individual cells or populations of cells (e.g., fixed cells or un-fixed cells), which may be represented visually or graphically, e.g., in a plot  1855   
     In  1820   b , the bead comprises nucleic acid barcode molecules that are releasably attached thereto, as described below. The bead may degrade or otherwise release the nucleic acid barcode molecules into the well  1802 ; the nucleic acid barcode molecules may then be used to barcode nucleic acid molecules within the well  1802 . Further processing may be performed either inside the partition or outside the partition. For example, one or more nucleic acid reactions may be performed, such as reverse transcription, nucleic acid extension, amplification, ligation, transposition, etc. In some instances, adapter sequences are ligated to the nucleic acid molecules, or derivatives thereof, as described elsewhere herein. For instance, sequencing primer sequences may be appended to each end of the nucleic acid molecule. In process  1850 , further characterization, such as sequencing may be performed to generate sequencing reads. The sequencing reads may yield information on individual cells or populations of cells (e.g., fixed cells or un-fixed cells), which may be represented visually or graphically, e.g., in a plot  1855 . 
     F. Additional Methods 
     The present disclosure provides methods and systems for multiplexing, and otherwise increasing throughput of samples (e.g., cells, fixed cells or un-fixed cells) for analysis. For example, a single or integrated process workflow may permit the processing, identification, and/or analysis of more or multiple analytes, more or multiple types of analytes, and/or more or multiple types of analyte characterizations. For example, in the methods and systems described herein, one or more labelling agents capable of binding to or otherwise coupling to one or more cells (e.g., cells, fixed cells or un-fixed cells) or cell features may be used to characterize cells and/or cell features. In some instances, cell features include cell surface features. Cell surface features may include, but are not limited to, a receptor, an antigen, a surface protein, a transmembrane protein, a cluster of differentiation protein, a protein channel, a protein pump, a carrier protein, a phospholipid, a glycoprotein, a glycolipid, a cell-cell interaction protein complex, an antigen-presenting complex, a major histocompatibility complex, an engineered T-cell receptor, a T-cell receptor, a B-cell receptor, a chimeric antigen receptor, a gap junction, an adherens junction, or any combination thereof. In some instances, cell features may include intracellular analytes, such as proteins, protein modifications (e.g., phosphorylation status or other post-translational modifications), nuclear proteins, nuclear membrane proteins, or any combination thereof. A labelling agent may include, but is not limited to, a protein, a peptide, an antibody (or an epitope binding fragment thereof), a lipophilic moiety (such as cholesterol), a cell surface receptor binding molecule, a receptor ligand, a small molecule, a bi-specific antibody, a bi-specific T-cell engager, a T-cell receptor engager, a B-cell receptor engager, a pro-body, an aptamer, a monobody, an affimer, a darpin, and a protein scaffold, or any combination thereof. The labelling agents can include (e.g., are attached to) a reporter oligonucleotide that is indicative of the cell surface feature to which the binding group binds. For example, the reporter oligonucleotide may comprise a barcode sequence (e.g., a reporter sequence) that permits identification of the labelling agent. For example, a labelling agent that is specific to one type of cell feature (e.g., a first cell surface feature) may have a first reporter oligonucleotide coupled thereto, while a labelling agent that is specific to a different cell feature (e.g., a second cell surface feature) may have a different reporter oligonucleotide coupled thereto. For a description of exemplary labelling agents, reporter oligonucleotides, and methods of use, see, e.g., U.S. Pat. No. 10,550,429; U.S. Pat. Pub. 20190177800; and U.S. Pat. Pub. 20190367969, each of which is herein entirely incorporated by reference for all purposes. 
     In a particular example, a library of potential cell feature labelling agents may be provided, where the respective cell feature labelling agents are associated with nucleic acid reporter molecules, such that a different reporter oligonucleotide sequence is associated with each labelling agent capable of binding to a specific cell feature. In other aspects, different members of the library may be characterized by the presence of a different oligonucleotide sequence label. For example, an antibody capable of binding to a first protein may have associated with it a first reporter oligonucleotide sequence, while an antibody capable of binding to a second protein may have a different reporter oligonucleotide sequence associated with it. The presence of the particular oligonucleotide sequence may be indicative of the presence of a particular antibody or cell feature which may be recognized or bound by the particular antibody. 
     For workflows comprising the use of fixation agents and/or un-fixing agents, labelling agents may be used to label samples (e.g., cells, fixed cells or un-fixed cells) at different points in time. In one embodiment, a plurality of cells is labeled prior to treatment with a fixation agent and/or after treatment with a fixation agent. In another embodiment, a plurality of fixed cells is labeled prior to treatment with an un-fixing agent and/or after treatment with an un-fixing agent. In one additional embodiment, a plurality of un-fixed cells is labeled prior to partitioning into partitions (e.g., wells or droplets) for further processing. In another embodiment, the methods, compositions, systems, and kits described herein provide labeled cells, labeled fixed cells or labeled un-fixed cells. 
     Labelling agents capable of binding to or otherwise coupling to one or more cells may be used to characterize a cell as belonging to a particular set of cells. For example, labeling agents may be used to label a sample of cells or a group of cells. In this way, a group of cells may be labeled as different from another group of cells. In an example, a first group of cells may originate from a first sample and a second group of cells may originate from a second sample. Labelling agents may allow the first group and second group to have a different labeling agent (or reporter oligonucleotide associated with the labeling agent). This may, for example, facilitate multiplexing, where cells of the first group and cells of the second group may be labeled separately and then pooled together for downstream analysis. The downstream detection of a label may indicate analytes as belonging to a particular group. 
     For example, a reporter oligonucleotide may be linked to an antibody or an epitope binding fragment thereof, and labeling a cell may comprise subjecting the antibody-linked barcode molecule or the epitope binding fragment-linked barcode molecule to conditions suitable for binding the antibody to a molecule present on a surface of the cell. The binding affinity between the antibody or the epitope binding fragment thereof and the molecule present on the surface may be within a desired range to ensure that the antibody or the epitope binding fragment thereof remains bound to the molecule. For example, the binding affinity may be within a desired range to ensure that the antibody or the epitope binding fragment thereof remains bound to the molecule during various sample processing steps, such as partitioning and/or nucleic acid amplification or extension, A dissociation constant (Kd) between the antibody or an epitope binding fragment thereof and the molecule to which it binds may be less than about 100 μM, 90 μM, 80 μM, 70 μM, 60 μM, 50 μM, 40 μM, 30 μM, 20 μM, 10 μM, 9 μM, 8 μM, 7 μM, 6 μM, 5 μM, 4 μM, 3 μM, 2 μM, 1 μM. 900 nM, 800 nM, 700 nM, 600 nM, 500 nM, 400 nM, 300 nM, 200 nM, 100 nM, 90 nM, 80 nM, 70 nM, 60 nM, 50 nM; 40 nM; 30 nM; 20 nM; 10 nM, 9 nM, 8 nM, 7 nM, 6 nM, 5 nM, 4 nM, 3 nM, 2 nM, 1 nM, 900 pM, 800 pM, 700 pM, 600 pM, 500 pM, 400 pM, 300 pM, 200 pM, 100 pM, 90 pM, 80 pM, 70 pM, 60 pM, 50 pM; 40 pM; 30 pM, 20 pM, 10 pM, 9 pM, 8 pM, 7 pM, 6 pM, 5 pM, 4 pM, 3 pM, 2 pM, or 1 pM, For example, the dissociation constant may be less than about 10 μM. 
     In another example, a reporter oligonucleotide may be coupled to a cell-penetrating peptide (CPP), and labeling cells may comprise delivering the CPP coupled reporter oligonucleotide into an analyte carrier. Labeling analyte carriers may comprise delivering the CPP conjugated oligonucleotide into a cell and/or cell bead by the cell-penetrating peptide. A CPP that can be used in the methods provided herein can comprise at least one non-functional cysteine residue, which may be either free or derivatized to form a disulfide link with an oligonucleotide that has been modified for such linkage. Non-limiting examples of CPPs that can be used in embodiments herein include penetratin, transportan, plsl, TAT(48-60), pVEC, MTS, and MAP. Cell-penetrating peptides useful in the methods provided herein can have the capability of inducing cell penetration for at least about 30%, 40%, 50%, 60%, 70%, 80%, 90%, 95%, 96%, 97%, 98%, 99%, or 100% of cells of a cell population. The CPP may be an arginine-rich peptide transporter. The CPP may be Penetratin or the Tat peptide. In another example, a reporter oligonucleotide may be coupled to a fluorophore or dye, and labeling cells may comprise subjecting the fluorophore-linked barcode molecule to conditions suitable for binding the fluorophore to the surface of the cell. In some instances, fluorophores can interact strongly with lipid bilayers and labeling cells may comprise subjecting the fluorophore-linked barcode molecule to conditions such that the fluorophore binds to or is inserted into a membrane of the cell. In some cases, the fluorophore is a water-soluble, organic fluorophore. In some instances, the fluorophore is Alexa 532 maleimide, tetramethylrhodamine-5-maleimide (TMR maleimide), BODIPY-TMR maleimide, Sulfo-Cy3 maleimide, Alexa 546 carboxylic acid/succinimidyl ester, Atto 550 maleimide, Cy3 carboxylic acid/succinimidyl ester, Cy3B carboxylic acid/succinimidyl ester, Atto 565 biotin, Sulforhodamine B, Alexa 594 maleimide, Texas Red maleimide, Alexa 633 maleimide, Abberior STAR 635P azide, Atto 647N maleimide, Atto 647 SE, or Sulfo-Cy5 maleimide. See, e.g., Hughes L D, et al. PLoS One. 2014 Feb. 4; 9(2):e87649, which is hereby incorporated by reference in its entirety for all purposes, for a description of organic fluorophores. 
     A reporter oligonucleotide may be coupled to a lipophilic molecule, and labeling cells may comprise delivering the nucleic acid barcode molecule to a membrane of a cell or a nuclear membrane by the lipophilic molecule. Lipophilic molecules can associate with and/or insert into lipid membranes such as cell membranes and nuclear membranes. In some cases, the insertion can be reversible. In some cases, the association between the lipophilic molecule, and the cell or nuclear membrane may be such that the membrane retains the lipophilic molecule (e.g., and associated components, such as nucleic acid barcode molecules, thereof) during subsequent processing (e.g., partitioning, cell permeabilization, amplification, pooling, etc.). The reporter nucleotide may enter into the intracellular space and/or a cell nucleus. In one embodiment, a reporter oligonucleotide coupled to a lipophilic molecule, will remain associated with and/or inserted into lipid membrane (as described herein) via the lipophilic molecule until lysis of the cell occurs, e.g., inside a partition. 
     A reporter oligonucleotide may be part of a nucleic acid molecule comprising any number of functional sequences, as described elsewhere herein, such as a target capture sequence, a random primer sequence, and the like, and coupled to another nucleic acid molecule that is, or is derived from, the analyte. 
     Prior to partitioning, the cells may be incubated with the library of labelling agents, that may be labelling agents to a broad panel of different cell features, e.g., receptors, proteins, etc., and which include their associated reporter oligonucleotides. Unbound labelling agents may be washed from the cells, and the cells may then be co-partitioned (e.g., into droplets or wells) along with partition-specific barcode oligonucleotides (e.g., attached to a support, such as a bead or gel bead) as described elsewhere herein. As a result, the partitions may include the cell or cells, as well as the bound labelling agents and their known, associated reporter oligonucleotides. 
     In other instances, e.g., to facilitate sample multiplexing, a labelling agent that is specific to a particular cell feature may have a first plurality of the labelling agent (e.g., an antibody or lipophilic moiety) coupled to a first reporter oligonucleotide and a second plurality of the labelling agent coupled to a second reporter oligonucleotide. For example, the first plurality of the labeling agent and second plurality of the labeling agent may interact with different cells, cell populations or samples, allowing a particular report oligonucleotide to indicate a particular cell population (or cell or sample) and cell feature. In this way, different samples or groups can be independently processed and subsequently combined together for pooled analysis (e.g., partition-based barcoding as described elsewhere herein). See, e.g., U.S. Pat. Pub. 20190323088, which is hereby entirely incorporated by reference for all purposes. 
     As described elsewhere herein, libraries of labelling agents may be associated with a particular cell feature as well as be used to identify analytes as originating from a particular cell population, or sample. Cell populations may be incubated with a plurality of libraries such that a cell or cells comprise multiple labelling agents. For example, a cell may comprise coupled thereto a lipophilic labeling agent and an antibody. The lipophilic labeling agent may indicate that the cell is a member of a particular cell sample, whereas the antibody may indicate that the cell comprises a particular analyte. In this manner, the reporter oligonucleotides and labelling agents may allow multi-analyte, multiplexed analyses to be performed. 
     In some instances, these reporter oligonucleotides may comprise nucleic acid barcode sequences that permit identification of the labelling agent which the reporter oligonucleotide is coupled to. The use of oligonucleotides as the reporter may provide advantages of being able to generate significant diversity in terms of sequence, while also being readily attachable to most biomolecules, e.g., antibodies, etc., as well as being readily detected, e.g., using sequencing or array technologies. 
     Attachment (coupling) of the reporter oligonucleotides to the labelling agents may be achieved through any of a variety of direct or indirect, covalent or non-covalent associations or attachments. For example, oligonucleotides may be covalently attached to a portion of a labelling agent (such a protein, e.g., an antibody or antibody fragment) using chemical conjugation techniques (e.g., Lightning-Link® antibody labelling kits available from Innova Biosciences), as well as other non-covalent attachment mechanisms, e.g., using biotinylated antibodies and oligonucleotides (or beads that include one or more biotinylated linker, coupled to oligonucleotides) with an avidin or streptavidin linker. Antibody and oligonucleotide biotinylation techniques are available. See, e.g., Fang, et al., “Fluoride-Cleavable Biotinylation Phosphoramidite for 5′-end-Labelling and Affinity Purification of Synthetic Oligonucleotides,” Nucleic Acids Res. Jan. 15, 2003; 31(2):708-715, which is entirely incorporated herein by reference for all purposes. Likewise, protein and peptide biotinylation techniques have been developed and are readily available. See, e.g., U.S. Pat. No. 6,265,552, which is entirely incorporated herein by reference for all purposes. Furthermore, click reaction chemistry such as a Methyltetrazine-PEG5-NHS Ester reaction, a TCO-PEG4-NHS Ester reaction, or the like, may be used to couple reporter oligonucleotides to labelling agents. Commercially available kits, such as those from Thunderlink and Abcam, and techniques common in the art may be used to couple reporter oligonucleotides to labelling agents as appropriate. In another example, a labelling agent is indirectly (e.g., via hybridization) coupled to a reporter oligonucleotide comprising a barcode sequence that identifies the label agent. For instance, the labelling agent may be directly coupled (e.g., covalently bound) to a hybridization oligonucleotide that comprises a sequence that hybridizes with a sequence of the reporter oligonucleotide. Hybridization of the hybridization oligonucleotide to the reporter oligonucleotide couples the labelling agent to the reporter oligonucleotide. In some embodiments, the reporter oligonucleotides are releasable from the labelling agent, such as upon application of a stimulus. For example, the reporter oligonucleotide may be attached to the labeling agent through a labile bond (e.g., chemically labile, photolabile, thermally labile, etc.) as generally described for releasing molecules from supports elsewhere herein. In some instances, the reporter oligonucleotides described herein may include one or more functional sequences that can be used in subsequent processing, such as an adapter sequence, a unique molecular identifier (UMI) sequence, a sequencer specific flow cell attachment sequence (such as an P5, P7, or partial P5 or P7 sequence), a primer or primer binding sequence, a sequencing primer or primer biding sequence (such as an R1, R2, or partial R1 or R2 sequence). 
     In some cases, the labelling agent can comprise a reporter oligonucleotide and a label. A label can be fluorophore, a radioisotope, a molecule capable of a colorimetric reaction, a magnetic particle, or any other suitable molecule or compound capable of detection. The label can be conjugated to a labelling agent (or reporter oligonucleotide) either directly or indirectly (e.g., the label can be conjugated to a molecule that can bind to the labelling agent or reporter oligonucleotide). In some cases, a label is conjugated to an oligonucleotide that is complementary to a sequence of the reporter oligonucleotide, and the oligonucleotide may be allowed to hybridize to the reporter oligonucleotide. 
       FIG. 19  describes exemplary labelling agents ( 1910 ,  1920 ,  1930 ) comprising reporter oligonucleotides ( 1940 ) attached thereto. Labelling agent  1910  (e.g., any of the labelling agents described herein) is attached (either directly, e.g., covalently attached, or indirectly) to reporter oligonucleotide  1940 . Reporter oligonucleotide  1940  may comprise barcode sequence  1942  that identifies labelling agent  1910 . Reporter oligonucleotide  1940  may also comprise one or more functional sequences  1943  that can be used in subsequent processing, such as an adapter sequence, a unique molecular identifier (UMI) sequence, a sequencer specific flow cell attachment sequence (such as an P5, P7, or partial P5 or P7 sequence), a primer or primer binding sequence, or a sequencing primer or primer biding sequence (such as an R1, R2, or partial R1 or R2 sequence). 
     Referring to  FIG. 19 , in some instances, reporter oligonucleotide  1940  conjugated to a labelling agent (e.g.,  1910 ,  1920 ,  1930 ) comprises a primer sequence  1941 , a barcode sequence  1942  that identifies the labelling agent (e.g.,  1910 ,  1920 ,  1930 ), and functional sequence  1943 . Functional sequence  1943  may be configured to hybridize to a complementary sequence, such as a complementary sequence present on a nucleic acid barcode molecule  1990  (not shown), such as those described elsewhere herein. In some instances, nucleic acid barcode molecule  1990  is attached to a support (e.g., a bead, such as a gel bead), such as those described elsewhere herein. For example, nucleic acid barcode molecule  1990  may be attached to the support via a releasable linkage (e.g., comprising a labile bond), such as those described elsewhere herein. In some instances, reporter oligonucleotide  1940  comprises one or more additional functional sequences, such as those described above. 
     In some instances, the labelling agent  1910  is a protein or polypeptide (e.g., an antigen or prospective antigen) comprising reporter oligonucleotide  1940 . Reporter oligonucleotide  1940  comprises barcode sequence  1942  that identifies polypeptide  1910  and can be used to infer the presence of an analyte, e.g., a binding partner of polypeptide  1910  (i.e., a molecule or compound to which polypeptide  1910  can bind). In some instances, the labelling agent  1910  is a lipophilic moiety (e.g., cholesterol) comprising reporter oligonucleotide  1940 , where the lipophilic moiety is selected such that labelling agent  1910  integrates into a membrane of a cell or nucleus. Reporter oligonucleotide  1940  comprises barcode sequence  1942  that identifies lipophilic moiety  1910  which in some instances is used to tag cells (e.g., groups of cells, cell samples, etc.) and may be used for multiplex analyses as described elsewhere herein. In some instances, the labelling agent is an antibody  1920  (or an epitope binding fragment thereof) comprising reporter oligonucleotide  1940 . Reporter oligonucleotide  1940  comprises barcode sequence  1942  that identifies antibody  1920  and can be used to infer the presence of, e.g., a target of antibody  1920  (i.e., a molecule or compound to which antibody  1920  binds). In other embodiments, labelling agent  1930  comprises an MHC molecule  1931  comprising peptide  1932  and reporter oligonucleotide  1940  that identifies peptide  1932 . In some instances, the MHC molecule is coupled to a support  1933 . In some instances, support  1933  may be a polypeptide, such as streptavidin, or a polysaccharide, such as dextran. In some instances, reporter oligonucleotide  1940  may be directly or indirectly coupled to MHC labelling agent  1930  in any suitable manner. For example, reporter oligonucleotide  1940  may be coupled to MHC molecule  1931 , support  1933 , or peptide  1932 . In some embodiments, labelling agent  1930  comprises a plurality of MHC molecules, (e.g., is an MHC multimer, which may be coupled to a support (e.g.,  1933 )). There are many possible configurations of Class I and/or Class II MHC multimers that can be utilized with the compositions, methods, and systems disclosed herein, e.g., MHC tetramers, MHC pentamers (MHC assembled via a coiled-coil domain, e.g., Pro5® MHC Class I Pentamers, (Prolmmune, Ltd.), MHC octamers, MHC dodecamers, MHC decorated dextran molecules (e.g., MHC Dextramer® (Immudex)), etc. For a description of exemplary labelling agents, including antibody and MHC-based labelling agents, reporter oligonucleotides, and methods of use, see, e.g., U.S. Pat. No. 10,550,429 and U.S. Pat. Pub. 20190367969, each of which is herein entirely incorporated by reference for all purposes. 
       FIG. 20  illustrates another example of a barcode carrying bead. In some embodiments, analysis of multiple analytes (e.g., RNA and one or more analytes using labelling agents described herein) may comprise nucleic acid barcode molecules as generally depicted in  FIG. 20 . In some embodiments, nucleic acid barcode molecules  2010  and  2020  are attached to support  2030  via a releasable linkage  2040  (e.g., comprising a labile bond) as described elsewhere herein. Nucleic acid barcode molecule  2010  may comprise adapter sequence  2011 , barcode sequence  2012  and adapter sequence  2013 . Nucleic acid barcode molecule  2020  may comprise adapter sequence  2021 , barcode sequence  2012 , and adapter sequence  2023 , wherein adapter sequence  2023  comprises a different sequence than adapter sequence  2013 . In some instances, adapter  2011  and adapter  2021  comprise the same sequence. In some instances, adapter  2011  and adapter  2021  comprise different sequences. Although support  2030  is shown comprising nucleic acid barcode molecules  2010  and  2020 , any suitable number of barcode molecules comprising common barcode sequence  2012  are contemplated herein. For example, in some embodiments, support  2030  further comprises nucleic acid barcode molecule  2050 . Nucleic acid barcode molecule  2050  may comprise adapter sequence  2051 , barcode sequence  2012  and adapter sequence  2053 , wherein adapter sequence  2053  comprises a different sequence than adapter sequence  2013  and  2023 . In some instances, nucleic acid barcode molecules (e.g.,  2010 ,  2020 ,  2050 ) comprise one or more additional functional sequences, such as a UMI or other sequences described herein. The nucleic acid barcode molecules  2010 ,  2020  or  2050  may interact with analytes as described elsewhere herein, for example, as depicted in  FIGS. 21A-C . 
     Referring to  FIG. 21A , in an instance where cells are labelled with labeling agents, sequence  2123  may be complementary to an adapter sequence of a reporter oligonucleotide. Cells may be contacted with one or more reporter oligonucleotide  2210  conjugated labelling agents  2110  (e.g., polypeptide, antibody, or others described elsewhere herein). In some cases, the cells may be further processed prior to barcoding. For example, such processing steps may include one or more washing and/or cell sorting steps. In some instances, a cell that is bound to labelling agent  2110  which is conjugated to oligonucleotide  2210  and support  2130  (e.g., a bead, such as a gel bead) comprising nucleic acid barcode molecule  2190  is partitioned into a partition amongst a plurality of partitions (e.g., a droplet of a droplet emulsion or a well of a microwell array). In some instances, the partition comprises at most a single cell bound to labelling agent  2110 . In some instances, reporter oligonucleotide  2210  conjugated to labelling agent  2110  (e.g., polypeptide, an antibody, pMHC molecule such as an MHC multimer, etc.) comprises a first adapter sequence  2111  (e.g., a primer sequence), a barcode sequence  2112  that identifies the labelling agent  2110  (e.g., the polypeptide, antibody, or peptide of a pMHC molecule or complex), and an adapter sequence  2113 . Adapter sequence  2113  may be configured to hybridize to a complementary sequence, such as sequence  2123  present on a nucleic acid barcode molecule  2190 . In some instances, oligonucleotide  2210  comprises one or more additional functional sequences, such as those described elsewhere herein. 
     Barcoded nucleic may be generated (e.g., via a nucleic acid reaction, such as nucleic acid extension or ligation) from the constructs described in  FIGS. 21A-C . For example, sequence  2113  may then be hybridized to complementary sequence  2123  to generate (e.g., via a nucleic acid reaction, such as nucleic acid extension or ligation) a barcoded nucleic acid molecule comprising cell (e.g., partition specific) barcode sequence  2122  (or a reverse complement thereof) and reporter sequence  2112  (or a reverse complement thereof). Barcoded nucleic acid molecules can then be optionally processed as described elsewhere herein, e.g., to amplify the molecules and/or append sequencing platform specific sequences to the fragments. See, e.g., U.S. Pat. Pub. 2018/0105808, which is hereby entirely incorporated by reference for all purposes. Barcoded nucleic acid molecules, or derivatives generated therefrom, can then be sequenced on a suitable sequencing platform. 
     In some instances, analysis of multiple analytes (e.g., nucleic acids and one or more analytes using labelling agents described herein) may be performed. For example, the workflow may comprise a workflow as generally depicted in any of  FIGS. 21A-C , or a combination of workflows for an individual analyte, as described elsewhere herein. For example, by using a combination of the workflows as generally depicted in  FIGS. 21A-C , multiple analytes can be analyzed. 
     In some instances, analysis of an analyte (e.g. a nucleic acid, a polypeptide, a carbohydrate, a lipid, etc.) comprises a workflow as generally depicted in  FIG. 21A . A nucleic acid barcode molecule  2190  may be co-partitioned with the one or more analytes. In some instances, nucleic acid barcode molecule  2190  is attached to a support  2130  (e.g., a bead, such as a gel bead), such as those described elsewhere herein. For example, nucleic acid barcode molecule  2190  may be attached to support  2130  via a releasable linkage  2140  (e.g., comprising a labile bond), such as those described elsewhere herein. Nucleic acid barcode molecule  2190  may comprise a barcode sequence  2121  and optionally comprise other additional sequences, for example, a UMI sequence  2122  (or other functional sequences described elsewhere herein). The nucleic acid barcode molecule  2190  may comprise a sequence  2123  that may be complementary to another nucleic acid sequence, such that it may hybridize to a particular sequence. 
     For example, sequence  2123  may comprise a poly-T sequence and may be used to hybridize to mRNA. Referring to  FIG. 21C , in some embodiments, nucleic acid barcode molecule  2190  comprises sequence  2123  complementary to a sequence of RNA molecule  2160  from a cell. In some instances, sequence  2123  comprises a sequence specific for an RNA molecule. Sequence  2123  may comprise a known or targeted sequence or a random sequence. In some instances, a nucleic acid extension reaction may be performed, thereby generating a barcoded nucleic acid product comprising sequence  2123 , the barcode sequence  2121 , UMI sequence  2122 , any other functional sequence, and a sequence corresponding to the RNA molecule  2160 . 
     In another example, sequence  2123  may be complementary to an overhang sequence or an adapter sequence that has been appended to an analyte. For example, referring to  FIG. 21B , in some embodiments, primer  2150  comprises a sequence complementary to a sequence of nucleic acid molecule  2160  (such as an RNA encoding for a BCR sequence) from an analyte carrier. In some instances, primer  2150  comprises one or more sequences  2151  that are not complementary to RNA molecule  2160 . Sequence  2151  may be a functional sequence as described elsewhere herein, for example, an adapter sequence, a sequencing primer sequence, or a sequence the facilitates coupling to a flow cell of a sequencer. In some instances, primer  2150  comprises a poly-T sequence. In some instances, primer  2150  comprises a sequence complementary to a target sequence in an RNA molecule. In some instances, primer  2150  comprises a sequence complementary to a region of an immune molecule, such as the constant region of a TCR or BCR sequence. Primer  2150  is hybridized to nucleic acid molecule  2160  and complementary molecule  2170  is generated. For example, complementary molecule  2170  may be cDNA generated in a reverse transcription reaction. In some instances, an additional sequence may be appended to complementary molecule  2170 . For example, the reverse transcriptase enzyme may be selected such that several non-templated bases  2180  (e.g., a poly-C sequence) are appended to the cDNA. In another example, a terminal transferase may also be used to append the additional sequence. Nucleic acid barcode molecule  2190  comprises a sequence  2124  complementary to the non-templated bases, and the reverse transcriptase performs a template switching reaction onto nucleic acid barcode molecule  2190  to generate a barcoded nucleic acid molecule comprising cell (e.g., partition specific) barcode sequence  2122  (or a reverse complement thereof) and a sequence of complementary molecule  2170  (or a portion thereof). In some instances, sequence  2123  comprises a sequence complementary to a region of an immune molecule, such as the constant region of a TCR or BCR sequence. Sequence  2123  is hybridized to nucleic acid molecule  2160  and a complementary molecule  2170  is generated. For example, complementary molecule  2170  may be generated in a reverse transcription reaction generating a barcoded nucleic acid molecule comprising cell (e.g., partition specific) barcode sequence  2122  (or a reverse complement thereof) and a sequence of complementary molecule  2170  (or a portion thereof). Additional methods and compositions suitable for barcoding cDNA generated from mRNA transcripts including those encoding V(D)J regions of an immune cell receptor and/or barcoding methods and composition including a template switch oligonucleotide are described in International Patent Application WO2013/075693, U.S. Patent Publication No. 2018/0105808, U.S. Patent Publication No. 2015/0376609, filed Jun. 26, 2015, and U.S. Patent Publication No, 2019/0367969, each of which applications is herein entirely incorporated by reference for all purposes. 
     EXAMPLES 
     Various features and embodiments of the disclosure are illustrated in the following representative examples, which are intended to be illustrative, and not limiting. Those skilled in the art will readily appreciate that the specific examples are only illustrative of the invention as described more fully in the claims which follow thereafter. Every embodiment and feature described in the application should be understood to be interchangeable and combinable with every embodiment contained within. 
     Example 1: RNA Expression Assay of Fixed PBMCs Using Un-Fixing Agents 
     This example illustrates preparation of a fixed biological sample of PBMCs, and the use of this fixed biological sample with four different catalytic un-fixing agents and RNA sequence assay reagents to determine cellular expression profiles. 
     Materials and Methods 
     A. Un-Fixing Agents 
     The un-fixing agents corresponding to compound (1) (Cat. No. 419443; Sigma-Aldrich Corp., St. Louis, Mo., USA), and compound (4) (Cat. No. 218766; Sigma-Aldrich Corp., St. Louis, Mo., USA) were purchased from Sigma-Aldrich and used without further purification. 
     The un-fixing agent of compound (8) was prepared using the following 2-step synthesis procedure. 
     Step 1: Diethyl (4-aminopyridin-3-yl)phosphonate. In step 1 the compound, diethyl (4-aminopyridin-3-yl)phosphonate was prepared according to the procedure described in Guilard, R. et al.  Synthesis,  2008, 10, 1575-1579. Briefly, to a solution of 3-bromopyridine-4-amine (2.5 g, 14.5 mmol, 1 equiv) (CAS:13534-98-0, Sigma Aldrich) in ethanol (58 mL) was added diethyl phosphite (2.2 mL, 17.3 mmol, 1.2 equiv.) triethylamine (3 mL, 1.5 equiv), PPh 3  (1.1 g, 4.3 mmol, 30 mol %) and Pd(OAc) 2  (0.39 g, 1.73 mmol, 12 mol %). The reaction mixture was purged with Argon for 5 min. After heating to reflux for 24 h, the reaction mixture was cooled to room T and conc. in vacuo. The residue was purified by silica gel chromatography (MeOH/DCM) to give the title compound (0.35 g, 11% yield).  1 H NMR (80 MHz, CDCl 3 ): δ=1.15 (t, 6H, CH 3 ), 4.18-3.69 (m, 4H, CH 2 ), 5.99 (br-s, 2H, NH 2 ), 6.49 (d, 1H), 8.03-7.93 (m, 1H), 8.22 (d, 1H). 
     Step 2: (4-Aminopyridin-3-yl)phosphonic acid (compound (8). In step 2, the target compound, (4-Aminopyridin-3-yl)phosphonic acid (compound (8)) was prepared by acid hydrolysis of the precursor compound of step 1. Diethyl (4-aminopyridin-3-yl)phosphonate (0.35 g, 1.52 mmol, 1 equiv) was suspended in 6 N HCl (aq.) (8 mL). After refluxing for 12 h, the reaction mixture was conc. in vacuo. The residue was washed with DCM, ether and conc in vacuo to afford the target compound (8) (247 mg, 93% yield).  1 H NMR (80 MHz, D 2 O): δ=6.85-6.55 (m, 1H), 8.05-7.94 (m, 1H), 8.40-8.26 (m, 1H). 
     The un-fixing agent of compound (11) was prepared using the following 4-step synthesis procedure. 
     Step 1. Methyl-4-amino-3-(diethoxyphosphoryl)benzoate. To a solution of methyl 4-amino-3-iodobenzoate (2 g, 7.2 mmol, 1 equiv) (CAS:19718-49-1, Sigma Aldrich) in acetonitrile (20 mL) was added triethyl phosphite (1.9 mL, 10.8 mmol, 1.5 equiv.) and Pd(OAc) 2  (0.16 g, 0.72 mmol, 10 mol %). The reaction mixture was purged with Argon for 5 min. After heating to reflux for 18 h, the reaction mixture was cooled to room temperature and conc. in vacuo. The residue was partitioned between ethyl acetate and water, and the organic layer was dried with MgSO4 and conc. in vacuo. The crude mixture was purified by silica gel chromatography (ethyl acetate/hexane) to give the title compound (1.4 g, 70% yield).  1 H NMR (500 MHz, DMSO-d 6 ): δ=1.27 (t, 6H, CH 3 ), 3.80 (s, 3H, OMe), 3.97-4.11 (m, 4H, OCH 2 ), 6.76 (br-s, 2H, NH 2 ), 6.80-6.83 (m, 1H), 7.82 (dd, 1H), 7.98 (dd, 1H). 
     Step 2. 4-Amino-3-(diethoxyphosphoryl)benzoic acid. To a solution of methyl-4-amino-3-(diethoxyphosphoryl)benzoate (0.96 g, 3.15 mmol, 1 equiv) in THF:methanol:water (10 mL:2.5 mL, ratio: 4:1:1) was added solid LiOH (0.45 g, 18.9 mmol, 6 equiv). After heating at 60° C. for 6 h, the reaction mixture was conc. in vacuo, acidified to pH 2 and solid precipitated out. The solid was filtered and washed twice with 1N HCl to give title compound (0.49 mg, 57% yield).  1 H NMR (80 MHz, CDCl 3 ): δ=1.34 (t, 6H, CH 3 ), 3.85-4.38 (m, 4H, OCH 2 ), 5.74 (br-s, 2H, NH 2 ), 6.50-6.76 (m, 1H), 7.86-8.36 (m, 2H). 
     Step 3. PEG-amide ethyl phosphonate. To a solution of 4-amino-3-(diethoxyphosphoryl)benzoic acid (0.25 g, 0.92 mmol, 1 equiv) and PEG-amine (0.75 g, 1.01 mmol, 1.1 equiv) in MeOH (4.6 mL) was added 4-(4,6-dimethoxy-1,3,5-triazin-2-yl)-4-methylmorpholinium chloride (DMTMM) under Argon. After stirring at room temperature for 18 h, the reaction mixture was conc. in vacuo, and the residue was partitioned between DCM and brine. The organic layer was washed with 1N HCl, saturated sodium bicarbonate solution, dried with MgSO4, filtered and conc. in vacuo to give the title compound (0.35 g, 36% yield) which was subjected without purification to the next step.  1 H NMR (80 MHz, CD 3 OD): δ=1.34 (t, 6H, CH 3 ), 3.44 (s, 3H, OCH3), 3.56-3.91 (m, PEG), 4.02-4.22 (m, 4H, OCH 2 ), 6.62-6.93 (m, 1H), 7.74-8.45 (m, 2H). 
     Step 4. PEG-amide phosphonic acid (compound (11)). PEG-amide ethyl phosphonate (0.35 g, 0.36 mmol, 1 equiv) was suspended in 6 N HCl (aq.) (8 mL). After refluxing for 12 h, the reaction mixture was conc. in vacuo. The residue was washed with MeOH, DCM and conc in vacuo to afford the title compound (Cat 9, 0.31 g, 94% yield).  1 H NMR (80 MHz, D 2 O): δ=3.02-4.06 (m, PEG), 7.36-7.52 (m, 1H), 7.99-8.09 (m, 1H). 
     Un-fixing agent solutions: the un-fixing agent of compounds (1), (4), (8), and (11) were each formulated in 30 mmol Tris buffer, pH 6.8 to a target concentration of 80 mM and the pH was adjusted to pH 6.5 using 2M NaOH. The final formulation was filtered through a 0.2 μm nylon syringe filter before addition into final composition of 50 mM of the un-fixing agent compound, 0.2% SDS, 10 mU/mL of Proteinase K and 0.2 U/mL RNAse inhibitor in 30 mM Tris, pH 6.8. 
     Bulk Un-fixing: PBMCs were fixed with 4% PFA for 24 h at 4° C. and quenched with 10% Fetal Bovine Serum (“FBS”) in PBS. The PFA-fixed PBMCs were treated with an un-fixing agent solution of compounds (1), (4), (8), or (11) formulated as described above at 40° C. for 2 h. Three different control samples also were prepared without un-fixing agent added: (1) fresh PBMCs (“Fresh Pellet” control); (2) PBMCs fixed with 4% PFA for 24 h then treated with 0.2% SDS, 10 mU/mL Proteinase K, and 0.2 U/mL RNAse inhibitor in 30 mM Tris, pH 6.8 at 40° C. for 2 h (“Fixed+PK, No UF agt.” control); and (3) PBMCs fixed with 4% PFA for 24 h (“fixed no un-fixing agent” control). 
     Bulk RNA isolation: After the bulk un-fixing treatment of the PFA-fixed PBMCs, the resulting treated (and control) samples were centrifuged at  450   g  for 5 min and pellet as well as supernatant were collected. RNA isolation from collected pellets and supernatants was performed using RNeasy Plus Mini Kit (Qiagen, Cat #_74134) and RNeasy MinElute Cleanup Kit (Qiagen, Cat #74204), respectively. Isolated RNA was quantified using Qubit™ RNA HS Assay Kit (Invitrogen, Cat # Q32855) and Agilent RNA ScreenTape System (Agilent Technologies). 
     Bulk RNA sequencing: RNA could not be isolated from the “fixed no un-fixing agent” control sample. Hence, this group was not tested via Bulk RNA sequencing. The “Fresh Pellet” control data was obtained by sequencing following bulk RNA isolation using the Qiagen kit without performing the 10× Genomics Single Cell 3′V3 protocol described below. For the four un-fixing agent treated samples and the remaining control samples described above, cDNA amplification was performed using an equivalent of 10 ng RNA. Post cDNA amplification, library prep was performed according to the 10× Genomics Single Cell 3′V3 protocol (10× Genomics, Pleasanton, Calif., USA). A 3000-cell load of fresh PBMCs was used as a single cell reference (“Fresh SC3P”) and library prep was performed using Single Cell 3′V3 protocol (10× Genomics, Pleasanton, Calif., USA). The final libraries were sequenced to between 25 and 100 million reads on a NovaSeq 6000 sequencer (Illumina Inc., San Diego, Calif., USA). Bulk library complexity was estimated using the software package Preseq, as described by Daley and Smith (see e.g., Daley and Smith, “Predicting the molecular complexity of sequencing libraries,” Nature Methods 10:325-327, 2013). Library complexity as used here refers to the estimated number of unique RNA molecules aligned properly to the transcriptome (i.e., reads considered to be informative and used for gene expression counting) as a function of all sequenced reads. Gene expression counts were down-sampled across libraries to match the lowest sequencing depth and pairwise gene expression correlations were computed as the Pearson correlation (R 2 ) of gene expression counts between samples. When comparing gene expression data from control, unfixed PBMCs, gene expression counts were summed across cells to produce pseudo-bulk gene expression counts, as is customary in commercial gene expression analysis software (e.g., 10× Genomics, Pleasanton, Calif., USA). 
     Results: The fixed PBMCs treated with an un-fixing agent of compound (1) or compound (4) exhibited increased RNA recovery per read sequenced relative to fixed PBMCs treated only with proteinase K and SDS. Further, as shown by the plot depicted in  FIG. 7 , treatment with the un-fixing agents of compound (1) (labeled above bars in plot as “Cmpd 1”) or compound (4) (labeled above bars in plot as “Cmpd 4”) also increased the transcriptomic mapping relative to the fixed PBMC samples treated with proteinase K but not treated with an un-fixing agent (labeled above bars in plot as “Fixed+PK, No UF agt.”). In contrast, treatment with the un-fixing agents of compound (8) (labeled above bars in plot as “Cmpd 8”) or compound (11) (labeled above bars in plot as “Cmpd 11”) decreased the transcriptomic mapping relative to fixed samples not treated with an un-fixing agent. 
     As shown in  FIG. 8 , treatment with the un-fixing agents of compounds (1), (4), and (8), resulted in higher complexity of the transcriptomic library relative to no treatment of the fixed PBMCs. Treatment with the un-fixing agent of compound (8) resulted in the highest library complexity. This increased library complexity increases the ability to detect rare transcripts from the fixed biological sample. 
     As shown by the results plotted in  FIG. 9 , treatment of fixed PBMCs with the un-fixing agent of compound (8) resulted in gene expression profiles that exhibited a high correlation to fresh cells (R 2 =0.882). As shown in  FIG. 10 , the fixed PBMCs not treated with an un-fixing agent exhibited a significantly lower correlation with the gene expression profiles of the fresh cells (R 2 =0.658). 
     In summary, PFA-fixed PBMC samples prepared for RNA gene expression profiling assay using a treatment with an un-fixing agent of compound (1) provided the highest recovery of RNA from the fixed biological sample. Treatment with the un-fixing agent of compound (8), however, provided gene expression profiles from the fixed cells that exhibited a higher correlation to the gene expression profiles obtained from fresh cells that were not fixed. 
     Example 2: RNA Expression Assay of Fixed Cells Using Un-Fixing Agent of Compound (1) and Low-Temperature Subtilisin a Protease Treatment 
     This example illustrates the use of the catalytic un-fixing agent of compound (1) in combination with a low-temperature Subtilisin A protease treatment to un-fix PFA-fixed Jurkats cells. 
     Materials and Methods: A protease stock solution of 100 mg/ml Subtilisin A from  Bacillus licheniformis  (Sigma-Aldrich, cat. #P5380) was prepared in H 2 O and stored at −20° C. A stock solution of the un-fixing agent of 100 mM compound (1) in 30 mM Tris-HCl, 1 mM EDTA, pH 6.8, was prepared and stored at room temperature. Dissociated single cells (Jurkats) were pelleted by 400 g centrifugation for 5 minutes and the supernatant removed. A fixing reagent solution of 4% PFA in PBS with 0.2 U/pL Qiagen RNAse Inhibitor (Qiagen, cat. #129916) was added to the pelleted cells and the mixture incubated at 4° C. overnight. The resulting fixed cells were quenched with 10% FBS in PBS and spun down for 5 minutes at 500 g, 4° C. 150,000 fixed cells were washed once in PBS then resuspended in 100 μL, 30 mM Tris-HCL, 1 mM EDTA, pH 6.8. RNAse Inhibitor was added to the fixed cell solution together with one of the following: (a) 5 mg/mL Subtilisin A; or (b) 5 mg/mL Subtilisin A and 50 mM compound (1). The fixed cell solution with Subtilisin A protease and with or without the un-fixing agent of compound (1) was allowed to incubate at 8° C. for 2 hours, followed by 15 minutes at 70° C. shaking continuously at 300 rpm on an Eppendorf Thermomixer. The resulting cell solutions were spun down for 5 minutes at 500 g, 4° C., and the supernatant and pellet fractions were collected separately. RNA extraction of the collected fractions was carried out using Qiagen 96 Kit (Qiagen, cat. #74181), bulk RNA sequencing, and/or single cell 3′ sequencing. Control samples of fresh cells, fresh cells with un-fixing conditions, and fixed cells without un-fixing treatment were also prepared and RNA extracted. RNA yield was assessed by Qubit HS Assay (Q32855) and yield and DV200 quality metric was assessed by Agilent 4200 High Sensitivity ScreenTape (5067-5579). 
     Results: Results are summarized in Table 2. Relative to RNA recovery from fresh cells, the use of a Subtilisin A protease treatment for 2 h at 8° C. allowed about 7% RNA recovery from 4% PFA-fixed Jurkats cells. The combination treatment of 50 mM of the un-fixing agent of compound (1) with the Subtilisin A protease for 2 h at 8° C., however, doubled the yield of RNA to about 15% of fresh, and improved the quality of recovered RNA as indicated by DV200. The further addition of a heating step of 15 min at 70° C. after the 2 h, 8° C. treatment to the combination treatment led to a substantially improved 71% recovery of RNA (relative to fresh) with &gt;95% DV200. 
     
       
         
           
               
               
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                   
                 Pellet  
                 Supernatant  
               
            
           
           
               
               
               
               
               
            
               
                   
                 Qubit  
                 Tapestation  
                 Qubit  
                 Tapestation  
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 Avg  
                 Avg  
                   
                 Avg  
                 Avg  
                   
               
               
                   
                 (SD)  
                 (SD)  
                 DV200  
                 (SD)  
                 (SD)  
                 DV200 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Fresh  
                 413  
                 (8)  
                 413  
                 (8)  
                 87.2  
                   
                   
                   
                   
                   
               
               
                 Subtilisin  
                 21  
                 (1)  
                 21  
                 (2)  
                 61.0  
                   
                   
                 8  
                 (3)  
                 52.5  
               
               
                 A  
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 8° C./2 hr  
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Subtilisin  
                   
                   
                 12  
                 (2)  
                 62.9  
                 43  
                 (4)  
                 54  
                 (7)  
                 86.5  
               
               
                 A +  
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Compound  
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 (1)  
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 8° C./2 hr  
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Subtilisin  
                 65  
                 (9)  
                 49  
                 (15)  
                 66.3  
                 230  
                 (17)  
                 246  
                 (22)  
                 95.2  
               
               
                 A +  
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Compound  
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 (1)  
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 8° C./2 hr  
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 70° C./  
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 15 min 
               
               
                   
               
            
           
         
       
     
     Example 3: Preparation of a Discrete Droplet Containing a Fixed Biological Sample and an Un-Fixing Agent 
     This example illustrates preparation of discrete droplets (GEMs) containing a fixed biological sample of PBMCs, and the un-fixing agent of compound (8), and then performing a single-cell RNA sequence expression profiling experiment using the un-fixed samples in the droplets. 
     Preparation of Fixed Biological Sample: 
     A fixed biological sample of fixed PBMCs is prepared as described above in Example 1. The fixed biological sample can be stored at 4° C. or −20° C. for several days or more before being processed in a droplet-based assay (e.g., a single cell assay). 
     Preparation of Un-Fixing Agent: 
     An 80 mM stock solution of the un-fixing agent of compound (8) is prepared as described in Example 1. 
     Generation of Droplets with Fixed Cells, Un-Fixing Agent, and Barcoded Gel-Beads 
     The fixed biological sample comprising fixed PBMCs is changed into the standard master mix used with the Chromium System (10× Genomics, Pleasanton, Calif., USA) for partitioning samples together with barcoded gel beads in discrete droplets called GEMs (“Gel Beads in Emulsion”). The Chromium System is prepared with the un-fixing agent solution added as a separate reagent in generating the GEM containing the sample PBMC and the barcode gel bead. Alternatively, the un-fixing agent solution is added to the reservoir containing the suspension of barcoded gel-beads and introduced into the GEMs through the same inlet channel with the gel-beads. Once generated, the GEMs are collected, and a heat incubation step is carried out. The heating step facilitates lysis and release of the cell contents, barcode oligonucleotides, and the reverse transcriptase (RT) catalyzed reaction that results in the cDNA synthesis reaction incorporating the barcodes in the 3′ synthons. In incorporating an un-fixing agent with the GEMs, the heat incubation step can be extended as necessary to allow for the un-fixing reaction catalyzed by compound (8) that removes the crosslinks from biomolecules released from the PBMC sample in the GEM. 
     Example 4: Bulk Un-Fixing of Fixed Cells Using Compounds (1) and/or (8) in Combination with Subtilisin a Protease Treatment 
     This example illustrates a study of the use of the catalytic un-fixing agents of compounds (1) and/or (8) in combination with protease, Subtilisin A at various temperatures, to un-fix PFA-fixed Jurkats cells and measure amounts and quality of the RNA from the treated cells into the pellet and/or supernatant. 
     Materials and Methods: 
     A. Protease preparation: A protease stock solution of 100 mg/ml Subtilisin A from  Bacillus licheniformis  (Sigma-Aldrich, cat. #P5380) was prepared in H 2 O and stored at −20° C. 
     B. Un-fixing agent stock solutions: A stock solution of the un-fixing agent of 100 mM compound (1) in 30 mM Tris-HCl, 1 mM EDTA, pH 6.8, was prepared and stored at room temperature. A stock solution of the un-fixing agent of 100 mM compound (8) in 30 mM Tris-HCl, 1 mM EDTA, pH 6.8, was prepared and stored at room temperature. 
     C. Fixed cell preparation: Dissociated single cells (Jurkats) were pelleted by 400 g centrifugation for 5 minutes and the supernatant removed. A fixing reagent solution of 4% PFA in PBS with 0.2 U/pL Qiagen RNAse Inhibitor (Qiagen, cat. #129916) was added to the pelleted cells and the mixture incubated at 4° C. overnight. The resulting fixed cells were quenched with 10% FBS in PBS and spun down for 5 minutes at 500 g, 4° C. 150,000 fixed cells were washed once in PBS then resuspended in 100 μL, 30 mM Tris-HCL, 1 mM EDTA, pH 6.8. 
     D. Cell un-fixing/protease treatment: RNAse Inhibitor was added to the fixed cell solution together with one of the following: (a) 5 mg/mL Subtilisin A protease; (b) 5 mg/mL Subtilisin A protease and 50 mM compound (1); (c) 5 mg/mL Subtilisin A protease and 50 mM compound (8); (d) 5 mg/mL Subtilisin A protease and 25 mM compound (8) and 25 mM compound (1). 
     The fixed cell solutions treated with Subtilisin A protease and with or without the un-fixing agents of compounds (1) and/or (8) were allowed to incubate under one of the following conditions: (1) 8° C. for 2 hours; (2) 8° C. for 2 hours, followed by 15 minutes at 70° C. shaking continuously at 300 rpm on an Eppendorf Thermomixer; (3) 53° C. for 45 min; or (4) 53° C. for 45 min followed by 15 minutes at 70° C. shaking continuously at 300 rpm on an Eppendorf Thermomixer. The resulting cell solutions were spun down for 5 minutes at 500 g, 4° C., and the supernatant and pellet fractions were collected separately. 
     E. RNA quantitation: RNA extraction of the collected fractions was carried out using Qiagen 96 Kit (Qiagen, cat. #74181), bulk RNA sequencing, and/or single cell 3′ sequencing. Control samples of fresh cells, fresh cells with un-fixing conditions, and fixed cells without un-fixing treatment were also prepared and RNA extracted. RNA yield was assessed by Qubit HS Assay (Q32855) and yield and DV200 quality metric was assessed by Agilent 4200 High Sensitivity ScreenTape (5067-5579). 
     Results: Results are summarized in Table 3. Relative to RNA recovery from fresh cells, the use of 50 mM compound (8) in combination with an incubation with Subtilisin A for 2 h at 8° C. followed by 15 min at 70° C. yielded 100% of RNA (relative to fresh) in the pellet. The quality of the RNA recovered in the pellet as indicated by DV200. The use of 50 mM compound (8) in combination with Subtilisin A incubation for 45 min at 53° C. also resulted in nearly all of the RNA recovered in the pellet, although the overall yield relative to fresh was significantly lower. The use of compound (1) alone or in combination with compound (8) and incubation with Subtilisin A at low or high temperature resulted in the recovery of a substantial portion (e.g., 40%-80%), but not all, of the RNA in the supernatant. 
     
       
         
           
               
               
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                   
                 Pellet  
                 Supernatant  
               
            
           
           
               
               
               
               
               
            
               
                   
                 Qubit  
                 Tapestation  
                 Qubit  
                 Tapestation  
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 Avg  
                 Avg  
                   
                 Avg  
                 Avg  
                   
               
               
                   
                 (SD)  
                 (SD)  
                 DV200  
                 (SD)  
                 (SD)  
                 DV200 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 Fresh  
                 1350  
                 (150)  
                 1805  
                 (460)  
                 90.7  
                   
                   
                 0  
                   
               
               
                 Subtilisin A  
                 108.4  
                 (0.6)  
                 49  
                 (3)  
                 74.1  
                   
                   
                 0  
                   
               
               
                 8° C./2 hr  
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Subtilisin A  
                 106  
                 (90)  
                 58  
                 (43)  
                 47.3  
                 398  
                 (260)  
                 231  
                 (204)  
                 60.5  
               
               
                 8° C./2 hr  
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 70° C./15 min  
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Subtilisin A  
                 144  
                 (18)  
                 244  
                 (43)  
                 47  
                 245  
                 (43)  
                 252  
                 (185)  
                 48  
               
               
                 53° C./45 min  
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Subtilisin A +  
                 167  
                 (10)  
                 96  
                 (33)  
                 44.5  
                 238  
                 (76)  
                 126  
                 (16)  
                 75.3  
               
               
                 Compound (1)  
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 8° C./2 hr  
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 70° C./15 min  
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Subtilisin A +  
                 113  
                 (10)  
                 151  
                 (26)  
                 73  
                 317  
                 (10)  
                 116  
                 (5)  
                 55  
               
               
                 Compound (1)  
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 53° C./45 min  
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Subtilisin A +  
                 1566  
                 (0)  
                 745  
                 (60)  
                 83.4  
                   
                   
                   
                   
                   
               
               
                 Compound (8)  
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 8° C./2 hr  
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 70° C./15 min  
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Subtilisin A +  
                 594  
                 (292)  
                 306  
                 (27)  
                 65  
                   
                   
                   
                   
                 56  
               
               
                 Compound (8)  
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 53° C./45 min  
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Subtilisin A +  
                 595  
                 (270)  
                 261  
                 (61)  
                 81.1  
                 492  
                 (17)  
                 290  
                 (20)  
                 92.4  
               
               
                 Compound (1) +  
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Compound (8)  
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 8° C./2 hr  
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 70° C./15 min  
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Subtilisin A +  
                 545  
                 (56)  
                 305  
                 (36)  
                 65.4  
                 410  
                 (53)  
                 224  
                 (25)  
                 87.0  
               
               
                 Compound (1) +  
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Compound (8)  
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 53° C./45 min  
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Subtilisin A +  
                 383  
                 (135)  
                 182  
                 (18)  
                 68.6  
                 669  
                 (134)  
                 266  
                 (48)  
                 79.4  
               
               
                 Compound (1) +  
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Compound (8)  
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 53° C./45 min  
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 70° C./15 min  
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                   
               
            
           
         
       
     
     Example 5: Bulk Un-Fixing of Fixed Cells Using Compounds (1) and/or (8) in Combination with Proteinase K Protease Treatment 
     This example illustrates a study of the use of the catalytic un-fixing agents of compounds (1) and/or (8) in combination with the protease, Proteinase K (“PK”) at various temperatures, to un-fix PFA-fixed Jurkats cells and measure amounts and quality of the RNA from the treated cells into the pellet and/or supernatant. 
     Materials and Methods: 
     A. Protease preparation: A stock solution of 20 mg/mL Proteinase K (Sigma-Aldrich, cat. #_AM258) was stored at −20° C. 
     B. Lin-fixing agent stock solutions: A stock solution of the un-fixing agent of 100 mM compound (1) in 30 mM Tris-HCl, 1 mM EDTA, pH 6.8, was prepared and stored at room temperature. A stock solution of the un-fixing agent of 100 mM compound (8) in 30 mM Tris-HCl, 1 mM EDTA, pH 6.8, was prepared and stored at room temperature. 
     C. Fixed cell preparation: Dissociated single cells (Jurkats) were pelleted by 400 g centrifugation for 5 minutes and the supernatant removed. A fixing reagent solution of 4% PFA in PBS with 0.2 U/pL Qiagen RNAse Inhibitor (Qiagen, cat. #129916) was added to the pelleted cells and the mixture incubated at 4° C. overnight. The resulting fixed cells were quenched with 10% FBS in PBS and spun down for 5 minutes at 500 g, 4° C. 150,000 fixed cells were washed once in PBS then resuspended in 100 μL, 30 mM Tris-HCL, 1 mM EDTA, pH 6.8. 
     D. Cell un-fixing/protease treatment: RNAse Inhibitor was added to the fixed cell solution together with one of the following: (a) 0.1 mg/mL Proteinase K protease; (b) 0.2 mg/mL Proteinase K protease; (c) 0.4 mg/mL Proteinase K protease; (d) 0.2 mg/mL Proteinase K protease and 50 mM compound (1); (e) 0.4 mg/mL Proteinase K protease and 50 mM compound (1); (f) 0.2 mg/mL Proteinase K protease and 50 mM compound (8); (g) 0.4 mg/mL Proteinase K protease and 50 mM compound (8); (h) 0.2 mg/mL Proteinase K protease and 25 mM compound (8) and 25 mM compound (1); or (i) 0.4 mg/mL Proteinase K protease and 25 mM compound (8) and 25 mM compound (1). The fixed cell solutions treated with Proteinase K protease and with or without the un-fixing agents of compounds (1) and/or (8) were allowed to incubate at 53° C. for 45 min. The resulting cell solutions were spun down for 5 minutes at 500 g, 4° C., and the supernatant and pellet fractions were collected separately. 
     E. RNA quantitation: RNA extraction of the collected fractions was carried out using Qiagen 96 Kit (Qiagen, cat. #74181), bulk RNA sequencing, and/or single cell 3′ sequencing. Control samples of fresh cells, fresh cells with un-fixing conditions, and fixed cells without un-fixing treatment were also prepared and RNA extracted. RNA yield was assessed by Qubit HS Assay (Q32855) and yield and DV200 quality metric was assessed by Agilent 4200 High Sensitivity ScreenTape (5067-5579). 
     Results: Results are summarized in Table 4. Relative to RNA recovery from fresh cells, the use of 50 mM compound (8) in combination with 0.2 or 0.4 mg/mL Proteinase K for 45 min at 53° C. yielded high quality RNA only in the pellet not in the supernatant. The use of 50 mM compound (1) alone, or in combination with compound (8) resulted in a significant portion (from 20%-80%), but not all, of the RNA released into the supernatant. 
     
       
         
           
               
               
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                   
                 Pellet  
                 Supernatant  
               
            
           
           
               
               
               
               
               
            
               
                   
                 Qubit  
                 Tapestation  
                 Qubit  
                 Tapestation  
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 Avg  
                 Avg  
                   
                 Avg  
                 Avg  
                   
               
               
                   
                 (SD)  
                 (SD)  
                 DV200  
                 (SD)  
                 (SD)  
                 DV200 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Fresh  
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Proteinase K  
                 1350  
                 (153)  
                 1804.5  
                 (465)  
                 90.695  
                 321.3  
                 (64)  
                 152.55  
                 (48)  
                 71.035  
               
               
                 at 0.1 mg/mL  
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 53° C./45 min  
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Proteinase K  
                 107.82  
                 (33)  
                 127.8  
                 (18)  
                 53.325  
                 251.1  
                 (42)  
                 121.05  
                 (16)  
                 70.12  
               
               
                 at 0.2 mg/mL  
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 53° C./45 min  
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Proteinase K  
                 106.2  
                 (38)  
                 76.725  
                 (82)  
                 45.4  
                 266.4  
                 (13)  
                 125.55  
                 (3)  
                 68.115  
               
               
                 at 0.4 mg/mL  
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 53° C./45 min  
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Compound (1) +  
                 136.8  
                 (17)  
                 316.35  
                 (255)  
                 51.875  
                 182.7  
                 (19)  
                 98.1  
                 (6)  
                 66.5  
               
               
                 Proteinase K  
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 at 0.2 mg/mL  
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 53° C./45 min  
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Compound (1) +  
                 59.85  
                 (10)  
                 62.685  
                 (2)  
                 53.73  
                 196.2  
                 (56)  
                 114.3  
                 (33)  
                 64.215  
               
               
                 Proteinase K  
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 at 0.4 mg/mL  
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 53° C./45 min  
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Compound (8) +  
                 56.43  
                 (15)  
                 48.15  
                 (16)  
                 43.3  
                   
                   
                   
                   
                   
               
               
                 Proteinase K  
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 at 0.2 mg/mL  
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 53° C./45 min  
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Compound (8) +  
                 368.1  
                 (144)  
                 145.35  
                 (43)  
                 76.09  
                   
                   
                   
                   
                   
               
               
                 Proteinase K  
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 at 0.4 mg/mL  
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 53° C./45 min  
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Compound (1) +  
                 480.6  
                 (59)  
                 166.05  
                 (17)  
                 74.88  
                 66.51  
                 (5)  
                 38.385  
                 (8)  
                 70.715  
               
               
                 Compound (8) +  
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Proteinase K  
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 at 0.2 mg/mL  
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 53° C./45 min  
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Compound (1) +  
                 546.3  
                 (67)  
                 297  
                 (154)  
                 80.175  
                 116.1  
                 (10)  
                 61.785  
                 (4)  
                 78.31  
               
               
                 Compound (8) +  
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Proteinase K  
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 at 0.4 mg/mL  
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 53° C./45 min 
               
               
                   
               
            
           
         
       
     
     Example 6: Bulk Sequencing of RNA from Cells Un-Fixed Using Treatment with Compounds (1) and/or (8) and Proteinase K 
     This example illustrates a study of bulk sequencing of RNA from PFA-fixed cells treated with un-fixing agents of compounds (1) and/or (8) and Proteinase K (“PK”) relative to RNA sequencing of fresh cells. 
     Materials and Methods: 
     A. Protease preparation: A stock solution of 20 mg/mL Proteinase K (Sigma-Aldrich, cat. #_AM258) was prepared in H 2 O and stored at −20° C. 
     B. Un-fixing agent stock solutions: A stock solution of the un-fixing agent of 100 mM compound (1) in 30 mM Tris-HCl, 1 mM EDTA, pH 6.8, was prepared and stored at room temperature. A stock solution of the un-fixing agent of 100 mM compound (8) in 30 mM Tris-HCl, 1 mM EDTA, pH 6.8, was prepared and stored at room temperature. 
     C. Fixed cell preparation: Dissociated single cells (Jurkats) were pelleted by 400 g centrifugation for 5 minutes and the supernatant removed. A fixing reagent solution of 4% PFA in PBS with 0.2 U/pL Qiagen RNAse Inhibitor (Qiagen, cat. #129916) was added to the pelleted cells and the mixture incubated at 4° C. overnight. The resulting fixed cells were quenched with 10% FBS in PBS and spun down for 5 minutes at 500 g, 4° C. 150,000 fixed cells were washed once in PBS then resuspended in 100 μL, 30 mM Tris-HCL, 1 mM EDTA, pH 6.8. 
     D. Cell un-fixing/protease treatment: RNAse Inhibitor was added to the fixed cell solution together with one of the following: (a) 0.1 mg/mL Proteinase K protease; (b) 0.1 mg/mL Proteinase K protease and 50 mM compound (1); (c) 0.1 mg/mL Proteinase K protease and 50 mM compound (8); (d) 0.1 mg/mL Proteinase K protease and 25 mM compound (1) and 25 mM compound (8). 
     The fixed cell solutions treated with Proteinase K protease and with or without the un-fixing agents of compounds (1) and/or (8) were allowed to incubate under one of the following conditions: (1) 8° C. for 2 hours; (2) 8° C. for 2 hours, followed by 15 minutes at 70° C. shaking continuously at 300 rpm on an Eppendorf Thermomixer; (3) 53° C. for 45 min; or (4) 53° C. for 45 min followed by 15 minutes at 70° C. shaking continuously at 300 rpm on an Eppendorf Thermomixer. The resulting cell solutions were spun down for 5 minutes at 500 g, 4° C., and the supernatant and pellet fractions were collected separately. 
     E. RNA Isolation: RNA extraction of the collected fractions was carried out using Qiagen 96 Kit (Qiagen, cat. #74181). Control samples of fresh cells, fresh cells with un-fixing conditions, and fixed cells without un-fixing treatment were also prepared and RNA extracted. RNA yield was assessed by Qubit HS Assay (Q32855) and yield and DV200 quality metric was assessed by Agilent 4200 High Sensitivity ScreenTape (5067-5579). 
     F. Bulk RNA sequencing: cDNA amplification of un-fixing agent treated and control samples was performed using an equivalent of 10 ng RNA. Bulk RNA was loaded in master mix in substitute for a single cell suspension, then GEM-RT, post cDNA amplification, and library prep was performed according to the 10× Genomics Single Cell 3′V3 protocol (10× Genomics, Pleasanton, Calif., USA). A 3000-cell load of fresh cells was used as a single cell reference (Fresh SC3P) and library prep was performed using Single cell 3′V3 protocol (10× Genomics, Pleasanton, Calif., USA). The final libraries were sequenced to between 25 and 100 million reads on a NovaSeq 6000 sequencer (Illumina Inc., San Diego, Calif., USA). Bulk library complexity was estimated using the software package Preseq, as described by Daley and Smith (see e.g., Daley and Smith, “Predicting the molecular complexity of sequencing libraries,” Nature Methods 10:325-327, 2013). Library complexity as used here refers to the estimated number of unique RNA molecules aligned properly to the transcriptome (i.e., reads considered to be informative and used for gene expression counting) as a function of all sequenced reads. Gene expression counts were down-sampled across libraries to match the lowest sequencing depth and pairwise gene expression correlations were computed as the Pearson correlation (R 2 ) of gene expression counts between samples. When comparing gene expression data from control, unfixed cells, gene expression counts were summed across cells to produce pseudo-bulk gene expression counts, as is customary in commercial gene expression analysis software (e.g., 10× Genomics, Pleasanton, Calif., USA). 
     Results: RNA quantitation and quality results are summarized in Table 5. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 5 
               
               
                   
               
               
                   
                   
                   
                 R 2  for Gene 
               
               
                   
                 cDNA 
                 Relative fraction of 
                 Expression 
               
               
                   
                 yield 
                 Max. No. UMIs to 
                 Fresh v. 
               
               
                   
                 (SD) 
                 Fresh (SD) 
                 Un-fixed 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 Fresh 
                 182 
                 1 
                 1 
               
               
                   
                 (20) 
                 (0.02) 
                   
               
               
                 0.1 mg/mL Proteinase K 
                 11 
                   
                   
               
               
                 53° C./45 min 
                 (1) 
                   
                   
               
               
                 Compound (1) + 
                 21 
                 0.48 
                   
               
               
                 0.1 mg/mL Proteinase K 
                 (4) 
                 (0.07) 
                   
               
               
                 53° C./45 min 
                   
                   
                   
               
               
                 Compound (8) + 
                 41 
                 0.85 
                 .79 
               
               
                 0.1 mg/mL Proteinase K 
                 (2) 
                 (0.01) 
                   
               
               
                 53° C./45 min 
                   
                   
                   
               
               
                 Compound (1) + 
                 37 
                 0.56 
                 .70 
               
               
                 Compound (8) + 
                 (20) 
                 (0.04) 
                   
               
               
                 0.1 mg/mL Proteinase K 
                   
                   
                   
               
               
                 53° C./45 min 
                   
                   
                   
               
               
                 Compound (1) + 
                 55 
                 0.68 
                 .72 
               
               
                 Compound (8) + 
                 (29) 
                 (0.03 
                   
               
               
                 Subtilisin A 
                   
                   
                   
               
               
                 8° C./2 h +70° C./15 min 
                   
                   
                   
               
               
                 Compound (8) + 
                 47 
                   
                   
               
               
                 Subtilisin A 
                 (1) 
                   
                   
               
               
                 53° C./45 min 
                   
                   
                   
               
               
                 Compound (8) + 
                 118 
                   
                   
               
               
                 Subtilisin A 
                 (1) 
                   
                   
               
               
                 53° C./45 min + 70° C./ 
                   
                   
                   
               
               
                 15 min 
               
               
                   
               
            
           
         
       
     
     Example 7: Bulk Un-Fixing with Compound (8) and Protease Followed by Single-Cell Partition Barcoding and cDNA Synthesis 
     This example illustrates a study of bulk un-fixing of PFA-fixed cells using compound (8) and Proteinase K (“PK”) following by partitioning into a GEM with barcoding and reverse transcription of the un-fixed RNA to provide cDNA. 
     Materials and Methods: 
     A. Protease preparation: A stock solution of 20 mg/mL Proteinase K (Sigma-Aldrich, cat. #AM258) was stored at −20° C. 
     B. Un-fixing agent of compound (8) stock solutions: A stock solution of the un-fixing agent of 100 mM compound (8) in 30 mM Tris-HCl, 1 mM EDTA, pH 6.8, was prepared and stored at room temperature. 
     C. Fixed cell preparation: Dissociated single cells (Jurkats) were pelleted by 400 g centrifugation for 5 minutes and the supernatant removed. A fixing reagent solution of 4% PFA in PBS with 0.2 U/pL Qiagen RNAse Inhibitor (Qiagen, cat. #129916) was added to the pelleted cells and the mixture incubated at 4° C. overnight. The resulting fixed cells were quenched with 10% FBS in PBS and spun down for 5 minutes at 500 g, 4° C. 150,000 fixed cells were washed once in PBS then resuspended in 100 μL, 30 mM Tris-HCL, 1 mM EDTA, pH 6.8. 
     D. Cell un-fixing/protease treatment: RNAse Inhibitor was added to the fixed cell solution together with 0.1 mg/mL Proteinase K and 50 mM compound (8). The fixed cell solution treated with the protease and compounds (8) were allowed to incubate at 53° C. for 45 min. The resulting cell solutions were spun down for 5 minutes at 500 g, 4° C., and the supernatant and pellet fractions were collected separately. Microscopic imaging showed that the cells un-fixed by this treatment remained intact although somewhat swollen relative to the fresh or PFA-fixed cells. 
     E. Partitioning of pellet fractions into GEMs and 3′-RT: pellet fractions collected from the un-fixing/protease treatment were centrifuged at 5 min 300 g and washed with PBS 0.1% BSA twice before loaded into the Single Cell 3′V3 protocol standard master mix used with the Chromium System (10× Genomics, Pleasanton, Calif., USA) for partitioning samples together with barcoded gel beads in discrete droplets called GEMs (“Gel Beads in Emulsion”). Once generated, the GEMs are collected, and a heat incubation step is carried out. The heating step facilitates release of the cell contents and RNA, capture of RNA by barcode oligonucleotides, and the reverse-transcription (RT) reaction that results in cDNA synthesis incorporating the barcodes in the 3′ synthons. 
     cDNA electropherogram analysis was performed using Agilent 2100 Bioanalyzer 5067-4626 to assess DNA size and yield from each sample. 
     Results: 
     Bulk un-fixing using a treatment with 0.1 mg/mL Proteinase K and the un-fixing agent of compound (8) has been shown to result in nearly 100% RNA content in pelleted cell fractions. Additionally, as noted above, this un-fixing treatment results in cells that appear to be intact although somewhat swollen relative to the fresh or PFA-fixed cells. Partitioning of the washed pellet fractions following this un-fixing treatment into GEMs together with reverse transcription (RT) master mix assay reagents was carried out under conditions for single-cell 3′-RT reaction. The cDNA electropherogram plot of Fresh cells partitioned RT assay reagent mix is shown in  FIG. 11A . The cDNA electropherogram plot of Fresh cells partitioned with the RT assay reagent mix and the protease, PK, is shown in  FIG. 11B . The plot of  FIG. 11A  shows the distinct mound profile with a peak between 1000 and 2500 indicating that the RT in the partition is capable of catalyzing the synthesis of full-length cDNA synthons. In contrast, the plot of  FIG. 11B  shows no mound profile indicating that the RT has not been able to synthesize cDNA. This lack of RT function is attributed to proteolytic deactivation of the RT by the protease, PK present in the partition.  FIG. 11C , however, where the fixed cells have been un-fixed in bulk using compound (8) and PK, then pelleted and washed, does show the distinct mound profile indicating that the RT in the partition remains active. The ability to un-fix the cells in bulk then pellet and wash them prior to partitioning effectively removes the protease and allows for single-cell 3′-RT reactions starting with the PFA-fixed cells. 
     Example 8: Bulk Un-Fixing of PFA-Fixed PBMCs with Compound (8) and a Cold-Active Protease Followed by Single-Cell Partition Barcoding and cDNA Synthesis 
     This example illustrates a study of bulk low-temperature un-fixing of PFA-fixed cells using the un-fixing agent of compound (8) and a cold-active protease (e.g., ArcticZymes Proteinase) at 25 C or 14 C, followed by protease deactivation, partitioning of un-fixed cells into GEMs with barcoding, and reverse transcription of the un-fixed RNA to provide cDNA. 
     Materials and Methods: 
     A. Protease preparation: A stock solution of 10 U/mL of the cold-active protease, ArcticZymes Proteinase (ArcticZymes Technologies ASA, Tromso, Norway) was stored at −20° C. 
     B. Un-fixing agent of compound (8) stock solutions: A stock solution of the un-fixing agent of 300 mM compound (8) in 50 mM Tris-HCl, 1 mM EDTA, pH 8.3, was prepared, filtered using a 5 μm syringe filter, and stored at room temperature. 
     C. Fixed cell preparation: Isolated single cells (PBMCs) were pelleted by 400 g centrifugation for 5 minutes and the supernatant removed. A fixing reagent solution of 4% PFA in PBS with 0.2 U/pL Qiagen RNAse Inhibitor (Qiagen, cat. #129916) was added to the pelleted cells and the mixture incubated at 4° C. overnight. The resulting fixed cells were quenched with RNAse-free 10% FBS (Seradigm 97069-085) in PBS and spun down for 5 minutes at 500 g, 4° C. 150,000 fixed cells were washed once in PBS then resuspended in 0.4% RNase free BSA in PBS with 20 U/mL RNase inhibitor. 
     D. Cell un-fixing/protease treatment: RNAse Inhibitor was added to the fixed cell solution together with 10 U/mL of the cold-active protease, ArcticZymes Proteinase, 50-200 mM of the un-fixing agent, compound (8), and 1 mM of the protease inhibitor, PMSF. The fixed cell solution treated with the protease and compound (8) was allowed to incubate at 14-25° C. for 45-90 min, followed by an incubation at 70-85° C. for 15 min. The resulting cell solution was spun down for 5 minutes at 500 g, 4° C., and the supernatant and pellet fractions were collected separately. Microscopic imaging showed that the cells un-fixed by this treatment remained intact although somewhat swollen relative to the fresh or PFA-fixed cells. 
     E. Partitioning of pellet fractions into GEMs and 3′-RT: pellet fractions collected from the un-fixing/protease treatment were centrifuged at 5 min 300 g and washed with PBS 0.04% BSA twice before loaded into the Single Cell 3′V3 protocol standard master mix used with the Chromium System (10× Genomics, Pleasanton, Calif., USA) for partitioning samples together with barcoded gel beads in discrete droplets called GEMs (“Gel Beads in Emulsion”). Once generated, the GEMs are collected, and a heat incubation step is carried out. The heating step facilitates release of the cell contents and RNA, capture of RNA by barcode oligonucleotides, and the reverse-transcription (RT) reaction that results in cDNA synthesis incorporating the barcodes in the 3′ synthons. 
     cDNA electropherogram analysis was performed using Agilent 2100 Bioanalyzer 5067-4626 to assess DNA size and yield from each sample. 
     Determination and mapping of PBMC cell types present in the samples was carried out as follows: PBMC cell type determination was performed by automated meta analysis of cell clusters identified using differentially expressed marker gene expression. PBMC cell type composition was identified by an automated script that quantifies the number and fraction of cell types known to be detected in PBMC samples by categorizing cells based on a combination of differentially expressed known marker genes for each cell type, with unclassified cells going to the undetermined category. 
     Results: 
     As noted above in Examples 5-7, bulk un-fixing using a combined treatment with protease and the un-fixing agent of compound (8) has been shown to result in nearly 100% RNA content of the treated cells remaining in the pelleted fractions. Additionally, as noted above, this un-fixing treatment results in cells that appear to be intact relative to the fresh or PFA-fixed cells. Partitioning of the washed pellet fractions into GEMs with reverse transcription (RT) master mix assay reagents following this un-fixing treatment allows for single-cell 3′-RT reaction to be carried out that produces cDNA closely resembling that produced by fresh cells. 
     As shown by the plots depicted in  FIGS. 12A, 12B and 12C , the cDNA synthons generated using the un-fixing treatment of both compound (8) and ArcticZymes Proteinase incubation at 14-25 C for 45-90 min, exhibited a cDNA electropherogram profile ( FIG. 12C ) that more closely resembling the profile for Fresh cells ( FIG. 12A ), than the profile from the treatment without compound (8) ( FIG. 12B ), which indicates little or no cDNA obtained from the sample. Additionally, the correlation of the gene expression values (R 2 ) for Fresh and Fixed+un-fixing treatment was substantially higher than for Fresh and Fixed without any un-fixing treatment. 
     Additionally, as shown by the plot depicted in  FIG. 13 , cell counting was carried out to determine the proportion of different PBMC cell types found in Fresh cells as compared to Fixed cells subjected to the un-fixing treatment. It was observed that the proportions of B cells, monocytes, T cells, and dendritic cells found in the Fresh cell sample was similar to the proportions found in the Fixed cell samples subjected to the un-fixing treatment. These comparative PBMC cell counting results indicate that the un-fixing treatment using a protease and an unfixing agent of the present disclosure can be used to recover relatively rare cell types from fixed samples and analyze these cell types in a droplet-based assay. 
     Example 9: Bulk Un-Fixing of PFA-Fixed Cells from Kidney Tissue Followed by Single-Cell Partition Barcoding and cDNA Synthesis 
     This example illustrates a study of bulk un-fixing of PFA-fixed cells from kidney tissue using the low temperature un-fixing treatment described in Example 7. 
     Materials and Methods: 
     A. Preparation of PFA-fixed cells from kidney tissue. Fresh kidney was rinsed with PBS minced into a fine tissue slurry using razor blades, then dissociated by incubation with an enzymatic mixture of 10 mg/ml pancreatin (Sigma, P1625) and 2.5 mg/ml collagenase A (Roche, 11 088 793 001) at 37 C shaken at 500 rpm for 30 minutes. The dissociated slurry was washed with 1 ml of autoMACS separation buffer twice at 500 rcf for 5 min then filtered through a 40 urn flowmi and pelleted for cell fixation with 4% PFA overnight at 4 C. The following day fixed cells were pelleted at 500 g for 5 minutes then neutralized with 3% RNase free BSA in PBS then pelleted again to be un-fixed with protease treatment. 
     B. Cell un-fixing/protease treatment: RNAse inhibitor was added to the fixed cell solution together with 10 U of the cold-active protease, ArcticZymes Proteinase, 100 mM of the un-fixing agent, compound (8), and 1 mM of the protease inhibitor, PMSF. The fixed cell solution treated with the protease and compound (8) was allowed to incubate at 14 or 25° C. for 90 min, followed by a higher temperature incubation at 70° C. for 15 min. The resulting cell solution was spun down for 5 minutes at 500 g, 4° C., and the supernatant and pellet fractions were collected separately. 
     C. Partitioning of pellet fractions into GEMs and 3′-RT was carried out as described in Example 8. 
     D. Cell counting was carried out as described in Example 8 to determine the proportions of cell types present in the kidney tissue samples following cDNA synthesis. 
     Results: As shown by the plot of results depicted in  FIG. 14 , the proportions of several different cell types, including B cells, dendritic cells, CD14, CD16, NK, platelets, various T cells, interstitial progenitors, proximal tubules, and distal nephrons, found in the PFA-fixed kidney tissue samples subjected to the un-fixing treatment, were very similar to the proportions of these cell types found in the fresh sample. These comparative cell proportion results indicate that the un-fixing treatment using a protease and an unfixing agent of the present disclosure can be used to recover relatively rare cell types from fixed tissue samples, such as differentiating nephrons and podocytes. 
     Example 10: Use of Proline Analog Un-fixing Agents of Compounds (12), (13), (14), (15) and a Cold-Active Protease for Bulk Un-fixing of PFA-fixed Jurkats 
     This example illustrates preparation of a fixed biological sample of Jurkats, and the un-fixing of this fixed biological sample using the proline analog un-fixing agents of compounds (12), (13), (14), and (15) in combination with protease. Proline is a unique amino acid that contains a secondary amine in a 5-membered ring, resulting in high nucleophilicity. The high nucleophilicity together with a proximal amine or acid moiety in the proline analog structures of compounds (12), (13), (14), and (15) suggests that these compounds, like the compounds (1)-(11) of the present disclosure, can be useful as catalytic un-fixing agents of PFA-fixed crosslinked biomolecules. 
     Materials and Methods 
     A. Proline Analog Un-Fixing Agent Preparation 
     The proline analog un-fixing agent corresponding to compound (12) ((2S,4R)-4-hydroxypyrrolidine-2-carboxylic acid) was purchased from Sigma-Aldrich and used without further purification (Cat. No. 51-35-4; Sigma-Aldrich Corp., St. Louis, Mo., USA). 
     The proline analog un-fixing agent of compound (13) ((2S,4R)-4-aminopyrrolidine-2-carboxylic acid) was prepared from a Boc protected reagent using the following 1-step procedure. 
     
       
         
         
             
             
         
       
     
     To a solid of (2S,4R)-4-amino-1-(tert-butoxycarbonyl)pyrrolidine-2-carboxylic acid (150 mg, 0.7 mmol, 1 equiv) (Catalog#: 132622-69-6, Combi-Blocks) was added 4 mL of 4M HCl and stirred at RT. After stirring at RT for 2 h, the reaction mixture was conc. in vacuo to give the title compound in quantitative yield. 1H NMR (80 MHz, CD 3 OD): δ=2.53-2.13 (m, 2H), 3.12-2.91 (m, 1H), 3.66-3.12 (m, 2H), 4.13-3.67 (m, 1H). 
     The proline analog un-fixing agent corresponding to compound (14) dihydrochloride ((2S,4S)-4-[(pyridin-4-yl)oxy]pyrrolidine-2-carboxylic acid dihydrochloride) was purchased from Enamine and used without further purification (Cat. No. EN300-7353434; Enamine, New Jersey, USA). 
     The proline analog un-fixing agent of compound (15) ((2S,4S)-4-(Pyridin-3-yloxy)pyrrolidine-2-carboxylic acid) was prepared from a Boc-protected reagent using the following 1-step procedure 
     
       
         
         
             
             
         
       
     
     To a solid of (2S,4S)-1-(tert-butoxycarbonyl)-4-(pyridin-3-yloxy)pyrrolidine-2-carboxylic acid (100 mg, 0.3 mmol, 1 equiv) (Catalog#: PH014404, Sigma Aldrich) was added 2 mL of 4M HCl and stirred at RT. After stirring at RT for 2 h, the reaction mixture was conc. in vacuo to give the title compound in quantitative yield. 1H NMR (80 MHz, CD 3 OD): δ=2.75-2.05 (m, 2H), 3.51 (br-s, 2H), 4.63-4.25 (m, 1H), 5.61-5.13 (m, 1H), 8.67-7.49 (m, 4H). 
     B. Un-fixing agent stock solutions: the proline analog un-fixing agents of compounds (12), (13), (14), and (15) were each formulated in 50 mmol Tris buffer, pH 8.3 to a target concentration of 300 mM and the pH was adjusted to pH 8.3 using 2M NaOH. The final formulation was filtered through a 0.2 μm nylon syringe filter before addition into final composition of 100 mM of the un-fixing agent compound, 10 U/mL ArcticZymes Proteinase and 0.2 U/mL RNAse inhibitor in 50 mM Tris, pH 8.3. 
     C. Protease stock solution: A stock solution of 10 U/mL of the cold-active protease, ArcticZymes Proteinase (ArcticZymes Technologies ASA, Tromso, Norway) was stored at −20° C. 
     D. Bulk un-fixing/protease treatment of fixed cells: Jurkats were fixed with 4% PFA for 24 h at 4° C. and quenched with 10% Fetal Bovine Serum (“FBS”) in PBS. RNAse inhibitor was added to the fixed cell solution together with 10 U/mL of the cold-active protease, ArcticZymes Proteinase, 1 mM of the protease inhibitor, and 100 mM of one of each of the tested un-fixing agents: compound (8), (12), (13), (14), or (15). The treated fixed cell solutions were allowed to incubate at 14-25° C. for 90 min, followed by a higher temperature incubation at 80° C. for 15 min. 
     E. Bulk RNA isolation: After the bulk un-fixing treatment of the PFA-fixed Jurkats, the resulting samples were centrifuged 5 minutes at 500 g, 4° C., and the supernatant and pellet fractions were collected separately. RNA isolation from collected pellets and supernatants was performed using RNeasy 96 Kit (Qiagen, Cat #_74181). Isolated RNA was quantified using Qubit™ RNA HS Assay Kit (Invitrogen, Cat # Q32855). 
     Results: As shown by the results in Table 6 below, the fixed Jurkats treated with the combination of ArcticZymes Proteinase and a proline analog un-fixing agent of compound (12), (13), (14), or (15), showed retention of RNA in the cell pellet that was comparable or better than the retention exhibited by the treatment with ArcticZymes Proteinase and compound (8). 
     
       
         
           
               
               
               
             
               
                 TABLE 6 
               
               
                   
               
               
                   
                 Pellet 
                 Supernatant 
               
               
                   
                 Avg.  
                 Avg.  
               
               
                   
                 RNA yield 
                 RNA yield 
               
               
                 Sample 
                 (ng) 
                 (ng) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 Fresh 
                 523 
                 2.4 
               
               
                 4% PFA 
                 4.2 
                 3.9 
               
               
                 4% PFA + AZ Proteinase 
                 32 
                 291 
               
               
                 4% PFA + Compound (8) 
                 54 
                 200 
               
               
                 4% PFA + AZ Proteinase + Compound (8) 
                 126 
                 180 
               
               
                 4% PFA + AZ Proteinase + Compound (12) 
                 140 
                 360 
               
               
                 4% PFA + AZ Proteinase + Compound (13) 
                 154 
                 339 
               
               
                 4% PFA + AZ Proteinase + Compound (14) 
                 158 
                 384 
               
               
                 4% PFA + AZ Proteinase + Compound (15) 
                 176 
                 360 
               
               
                   
               
            
           
         
       
     
     Example 11: Bulk Un-Fixing of Antibody-Labeled PFA-Fixed PBMCs with Compound (8) and a Cold-Active Protease Followed by Single-Cell Partition Barcoding 
     This example illustrates a study of bulk low-temperature un-fixing of stained PFA-fixed cells using the un-fixing agent of compound (8) and a cold-active protease (e.g., ArcticZymes Proteinase) at 25° C., followed by protease deactivation, partitioning of un-fixed cells into GEMs with barcoding, and processing of the un-fixed cells using feature barcoding technology protocols (10× Genomics, Inc.). 
     Materials and Methods: 
     Preparation of reagents: The protease and the un-fixing agent of compound (8) stock solutions were prepared according to Example 8. 
     Labeling Agent: The Immune Profiling (Biolegend) antibody panel (T cell, B cell, monocyte and NK cell-identifying antibodies) were used to label PBMCs. 
     Fixed cell preparation: Isolated single cells (PBMCs) prepared as described in Example 8 with the exception of staining with TotalSeq C antibodies before or after overnight fixation with 4% PFA. 
     Cell un-fixing/protease treatment: Cells were treating with un-fixing agent of compound (8) and cold-active protease as described in Example 8. 
     Partitioning of pellet fractions into GEMs and 5′ feature barcoding workflow: pellet fractions collected from the un-fixing/protease treatment were centrifuged at 5 min 500 g and washed with PBS 0.04% BSA twice before loading into the master mix provided for the Single Cell V(D)J Reagent Kits with Feature Barcoding technology for Cell Surface Protein for use with the Chromium System (10× Genomics Inc., Pleasanton, Calif., USA) for partitioning samples together with barcoded gel beads in discrete droplets called GEMs (“Gel Beads in Emulsion”). Once generated, the GEMs are collected, and a heat incubation step is carried out. The heating step facilitates release of the cell contents and RNA, capture of RNA and the TotalSeq C oligonucleotides by barcode oligonucleotides, and the reverse-transcription (RT) reaction that results in cDNA synthesis incorporating the barcodes in the 5′ synthons, as well as generation of barcoded extension products derived from the TotalSeq C oligonucleotides. 
     Results: Antibody-stained PBMCs that underwent PFA fixation and un-fixing treatment with compound (8) and the cold-active protease, ArcticZymes Proteinase (Fresh+Ab+Fix) showed similar density profiles to fresh PBMC controls, indicating the un-fixing treatment process was compatible with antibody staining workflows. PFA fixation followed by antibody staining (Fix+Ab) showed lower antibody counts per cell. However, the population profile for individual cell types was maintained even after fixation and un-fixing treatment ( FIG. 23 ), with a high gene expression correlation when compared to fresh controls ( FIG. 22 ).  FIG. 22  shows gene expression correlations computed as the Pearson correlation (R 2 ) of gene expression counts between samples. Freshly stained and freshly stained then fixed samples showed R 2 =0.743, while freshly stained and fixed then stained PBMCs showed R 2 =0.813.  FIG. 23  shows that PBMC cell type counts determined by gene expression profile are relatively similar between freshly stained (3), freshly stained and fixed (2), and fixed and stained (1) samples with the exception of a high undetermined cell category in the freshly stained and fixed condition. 
     Notwithstanding the foregoing description or the appended claims, the disclosure set forth herein is also defined by the following numbered clauses, which may be beneficial alone or in combination, with one or more other causes or embodiments. Each of these individually numbered clauses may be used or combined with any of the preceding or following clauses. Thus, these clauses are intended to provide support for all such combinations and is not necessarily limited to specific combinations explicitly provided below:
     1. A composition comprising a fixed biological sample and an un-fixing agent encapsulated in a discrete droplet.   2. The composition of clause 1, wherein the fixed biological sample is derived from a tissue sample, a biopsy sample, or a blood sample.   3. The composition of clauses 1-2, wherein the fixed biological sample is a single cell.   4. The composition of clauses 1-3, wherein the fixed biological sample has been fixed with a fixing reagent selected from paraformaldehyde.   5. The composition of clauses 1-4, wherein the discrete droplet further comprises a bead.   6. The composition of clause 5, wherein the un-fixing agent is contained in a bead.   7. The composition of clause 6, wherein the discrete droplet further comprises assay reagents; optionally, wherein the assay reagents are contained in a gel bead.   8. The composition of clause 6, wherein the discrete droplet further comprises a barcode optionally, wherein the barcode is contained in a gel bead.   9. The composition of clauses 1-8, wherein the un-fixing agent is capable of removing crosslinks formed in biomolecules by fixation with an aldehyde (e.g., paraformaldehyde, glutaraldehyde), an NHS ester (e.g., N-Hydroxysuccinimide), an imidoesters, or a combination thereof.   10. The composition of clauses 1-9, wherein the un-fixing agent is capable of removing crosslinks formed in biomolecules by fixation with paraformaldehyde; optionally, fixation with a PF solution at a concentration of 1%-4% PFA.   11. The composition of clauses 1-10, wherein the un-fixing agent comprises a compound selected from compound (8), compound (1), compound (2), compound (3), compound (4), compound (5), compound (6), compound (7), compound (9), compound (10), compound (11), compound (12), compound (13), compound (14), compound (15), and a combination thereof   

     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
         
         12. A method for preparing a biological sample comprising: generating a discrete droplet encapsulating a fixed biological sample and an un-fixing agent. 
         13. The method of clause 12, wherein the method further comprises fixing the biological sample prior to generating the discrete droplet. 
         14. The method of any one of clauses 12-13, wherein the fixed biological sample is derived from a tissue sample, a biopsy sample, or a blood sample. 
         15. The method of any one of clauses 12-14, wherein the fixed biological sample is a single cell. 
         16. The method of any one of clauses 12-15, wherein the amount of time prior to generating the discrete droplet when the biological sample is fixed is at least 1 hour, at least 2 hours, at least 6 hours, at least 12 hours, at least 24 hours, at least 1 week, at least 1 month, at least 6 months, or longer. 
         17. The method of any one of clauses 12-16, wherein the method further comprises heating the discrete droplet. 
         18. The method of any one of clauses 12-17, wherein the fixed biological sample has been fixed with a fixing reagent selected from an aldehyde (e.g., paraformaldehyde, glutaraldehyde), an NHS ester (e.g., N-Hydroxysuccinimide), an imidoesters, or a combination thereof. 
         19. The method of clause 18, wherein the fixing reagent is paraformaldehyde (“PFA”); optionally, the fixing reagent is a PFA solution at a concentration of 1%-4% PFA 
         20. The method of any one of clauses 12-19, wherein the discrete droplet further comprises a bead. 
         21. The method of clause 20, wherein the un-fixing agent is contained in the bead. 
         22. The method of any one of clauses 12-21, wherein the discrete droplet further comprises assay reagents; optionally, wherein the assay reagents are contained in a bead. 
         23. The method of any one of clauses 12-22, wherein the discrete droplet further comprises a barcode; optionally, wherein the barcode contained in a bead. 
         24. The method of any one of clauses 12-23, wherein the un-fixing agent is capable of removing crosslinks formed in biomolecules by fixation with an aldehyde (e.g., paraformaldehyde, glutaraldehyde), an NHS ester (e.g., N-Hydroxysuccinimide), an imidoesters, or a combination thereof. 
         25. The method of any one of clauses 12-24, wherein the un-fixing agent is capable of removing crosslinks formed in biomolecules by fixation with paraformaldehyde; optionally, fixation with a PFA solution at a concentration of 1%-4% PFA. 
         26. The method of any one of clauses 12-25, wherein the un-fixing agent comprises a compound selected from compound (8), compound (1), compound (2), compound (3), compound (4), compound (5), compound (6), compound (7), compound (9), compound (10), compound (11), compound (12), compound (13), compound (14), compound (15), and a combination thereof. 
         27. An assay method comprising: (a) generating a discrete droplet encapsulating a fixed biological sample, an un-fixing agent, and assay reagents; and (b) detecting analytes from the reaction of the assay reagents and the un-fixed biological sample. 
         28. The method of clause 27, wherein the method further comprises fixing the biological sample prior to generating the discrete droplet. 
         29. The method of any one of clauses 27-28, wherein the fixed biological sample is derived from a tissue sample, a biopsy sample, or a blood sample. 
         30. The method of any one of clauses 27-30, wherein the fixed biological sample is a single cell. 
         31. The method of any one of clauses 27-30, wherein the method further comprises heating the discrete droplet. 
         32. The method of any one of clauses 27-31, wherein the assay reagents are contained in a bead. 
         33. The method of any one of clauses 27-32, wherein the assay reagents comprise a barcode. 
         34. The method of any one of clauses 27-33, wherein detecting analytes from the reaction of the assay reagents comprises determining populations of nucleic acid sequences comprising a barcode obtained from the discrete droplet. 
         35. A composition comprising a compound selected from compound (8), compound (9), compound (10), and a combination thereof: 
       
    
     
       
         
         
             
             
         
       
         
         36. The composition of clause 35 further comprising a fixed biological sample; optionally, wherein the fixed biological sample is derived from a tissue sample, a biopsy sample, or a blood sample. 
         37. The composition of clause 35 further comprising a protease; optionally, wherein the protease is a thermolabile protease or cold-active protease; optionally, wherein the protease is selected from Subtilisin A, Proteinase K, ArcticZymes Proteinase, and a combination thereof. 
         38. The composition of any one of clauses 35-37, wherein the composition is within a partition; optionally, wherein the partition is a discrete droplet. 
         39. A kit comprising: assay reagents; and an un-fixing agent composition comprising a compound selected from compound (8), compound (1), compound (2), compound (3), compound (4), compound (5), compound (6), compound (7), compound (9), compound (10), compound (11), compound (12), compound (13), compound (14), compound (15), and a combination thereof. 
         40. The kit of clause 39 further comprising a protease; optionally, wherein the protease is a thermolabile protease or cold-active protease; optionally, wherein the protease is selected from Subtilisin A, Proteinase K, ArcticZymes Proteinase, and a combination thereof. 
         41. The kit of clause 39, wherein the un-fixing agent composition is contained in a bead. 
         42. The kit of any one of clauses 39-41, wherein the assay reagents are contained in a bead. 
         43. The kit of any one of clauses 39-42, wherein the assay reagents comprise a barcode. 
         44. The kit of any one of clauses 39-43, wherein the kit further comprises a fixing reagent; optionally, wherein the fixing reagent is selected from an aldehyde (e.g., paraformaldehyde, glutaraldehyde), an NHS ester (e.g., N-Hydroxsuccinimide), an imidoester, or a combination thereof; optionally, wherein the fixing reagent is a PFA solution at a concentration of 1%-4% PFA. 
         45. An assay method comprising: (a) incubating a fixed cell with an un-fixing solution comprising an un-fixing agent and a protease, thereby generating an un-fixed cell; (b) separating the un-fixed cell from the un-fixing solution; (c) combining the un-fixed cell with assay reagents; and (d) detecting analytes from the reaction of the assay reagents. 
         46. The assay method of clause 45, wherein the fixed cells have been fixed with paraformaldehyde (“PFA”); optionally, fixed with PFA at a concentration of 1%-4%. 
         47. The assay method of clause 45, wherein the un-fixing agent is a composition comprising a compound selected from compound (8), compound (1), compound (2), compound (3), compound (4), compound (5), compound (6), compound (7), compound (9), compound (10), compound (11), compound (12), compound (13), compound (14), compound (15), and a combination thereof. 
         48. The assay method of clause 45, wherein the un-fixing agent is a composition comprises compound (8) at a concentration of about 1 mM to about 500 mM, about 50 mM to about 300 mM, or about 25 mM to about 200 mM; optionally, wherein the concentration is about 50 mM to about 200 mM. 
         49. The assay method of clause 45, wherein the protease is selected from Subtilisin A, Proteinase K, ArcticZymes Proteinase, and a combination thereof. 
         50. The assay method of clause 45, wherein incubating the fixed cell with the un-fixing solution is for 30-120 min at a temperature of from about 15 C to 60 C; optionally, wherein the incubating is for at least 90 minutes at 25 C. 
         51. The assay method of clause 45, wherein the method further comprises incubating with a protease inhibitor at a temperature of from about 60 C to about 70 C for about 10 to about 
         20 min; optionally, wherein the protease inhibitor is PMSF at a concentration of about 1 mM. 
         52. The assay method of clause 45, wherein incubating the fixed cell with the un-fixing solution is for 30-60 min at a temperature of from about 50 C to 60 C; optionally, wherein the incubating is for at least 45 minutes at 53 C. 
         53. The assay method of clause 45, wherein separating the un-fixed cell from the un-fixing solution comprises centrifuging the solution to provide a pellet of un-fixed cells. 
         54. The assay method of clause 50, wherein the separating further comprises resuspending the pellet of un-fixed cells in a solution. 
         55. The method of clause 45, wherein the assay reagents comprise a reverse transcriptase; optionally, wherein the assay reagents further comprise reagents for cDNA synthesis. 
         56. The assay method of clause 45, wherein combining the un-fixed cell with assay reagents further comprises generating a discrete droplet encapsulating the un-fixed cell and assay reagents. 
         57. The method of clause 52, wherein the discrete droplet further comprises a barcode, whereby RNA of the un-fixed cell is labeled by the barcode. 
         58. A method for analysis of fixed cells comprising
       (a) providing a plurality of fixed cells, wherein a fixed cell of said plurality of fixed cells comprises a plurality of crosslinked nucleic acid molecules;   (b) un-fixing said fixed cell with an un-fixing agent to provide an un-fixed cell comprising a plurality of un-crosslinked nucleic acid molecules from said plurality of crosslinked nucleic acid molecules;   (c) generating a plurality of barcoded nucleic acid molecules from said plurality of un-crosslinked nucleic acid molecules and a plurality of nucleic acid barcode molecules, wherein a barcoded nucleic acid molecule of said plurality of barcoded nucleic acid molecules comprises i) a sequence corresponding to an un-crosslinked nucleic acid molecule of said plurality of said un-crosslinked nucleic acid molecules or a complement thereof, and ii) a barcode sequence or a complement thereof.   
     
         59. The method of clause 58, wherein (c) is performed in a plurality of partitions. 
         60. The method of clause 59, wherein said plurality of partitions is a plurality of droplets. 
         61. The method of clause 59, wherein said plurality of partitions is a plurality of wells. 
         62. The method of clauses 59 or 60, wherein a partition of said plurality of partitions comprises said un-fixed cell and a support comprising said plurality of nucleic acid barcode molecules. 
         63. The method of clause 62, wherein said support is a bead. 
         64. The method of clause 58, wherein said barcode sequence is a partition-specific barcode sequence. 
         65. The method of clause 58, wherein said plurality of fixed cells is a plurality of paraformaldehyde fixed cells. 
         66. The method of clause 58, wherein the un-fixing agent is capable of removing crosslinks formed in biomolecules by fixation with an aldehyde, an NHS ester (e.g., N-Hydroxysuccinirnide), an imidoesters, or a combination thereof. 
         67. The method of clause 58, wherein the un-fixing agent is capable of removing crosslinks formed in biomolecules by fixation with paraformaldehyde. 
         68. The method of clause 58, wherein said un-fixing agent comprises a compound selected from compound (8), compound (1), compound (2), compound (3), compound (4), compound (5), compound (6), compound (7), compound (9), compound (10), compound (11), compound (12), compound (13), compound (14), compound (15), and a combination thereof. 
         69. The method of clause 68, wherein said un-fixing agent further comprises a protease. 
         70. The method of clause 69, wherein said protease is a thermolabile protease. 
         71. The method of clause 69, wherein said protease is a cold-active protease. 
         72. The method of clause 58, wherein said plurality of crosslinked nucleic acid molecules comprises cross-linked ribonucleic acid (RNA) molecules. 
         73. The method of clause 58, wherein said plurality of un-crosslinked nucleic acid molecules comprises un-crosslinked ribonucleic acid (RNA) molecules. 
         74. The method of clause 58, wherein said sequence corresponding to an un-crosslinked nucleic acid molecule is a sequence corresponding to an un-crosslinked RNA molecule. 
         75. The method of clause 58, wherein said fixed cell comprises a labeling agent. 
         76. The method of clause 75, wherein said labeling agent is selected from the group consisting of protein, a peptide, an antibody, a lipophilic moiety, a cell surface receptor binding molecule, a receptor ligand, a small molecule, a bi-specific antibody, a bi-specific T-cell engager, a T-cell receptor engager, a B-cell receptor engager, a pro-body, an aptamer, a monobody, an affimer, a darpin, a protein scaffold, and any combination thereof. 
         77. The method of clause 75, wherein said labeling agent comprises a reporter oligonucleotide. 
         78. The method of clause 77, wherein said reporter oligonucleotide comprises a reporter sequence that identifies the labeling agent. 
         79. The method of clause 78, further comprising generating an additional barcoded nucleic acid molecule from said reporter oligonucleotide and said plurality of nucleic acid barcode molecules, wherein said additional barcoded nucleic acid molecule of said plurality of barcoded nucleic acid molecules comprises i) said reporter sequence or a complement thereof and ii) said barcode sequence of complement thereof. 
       
    
     While the foregoing disclosure of the present invention has been described in some detail by way of example and illustration for purposes of clarity and understanding, this disclosure including the examples, descriptions, and embodiments described herein are for illustrative purposes, are intended to be exemplary, and should not be construed as limiting the present disclosure. It will be clear to one skilled in the art that various modifications or changes to the examples, descriptions, and embodiments described herein can be made and are to be included within the spirit and purview of this disclosure and the appended claims. Further, one of skill in the art will recognize a number of equivalent methods and procedure to those described herein. All such equivalents are to be understood to be within the scope of the present disclosure and are covered by the appended claims. 
     Additional embodiments of the invention are set forth in the following claims. 
     The disclosures of all publications, patent applications, patents, or other documents mentioned herein are expressly incorporated by reference in their entirety for all purposes to the same extent as if each such individual publication, patent, patent application or other document were individually specifically indicated to be incorporated by reference herein in its entirety for all purposes and were set forth in its entirety herein. In case of conflict, the present specification, including specified terms, will control.