Patent Publication Number: US-2022220549-A1

Title: Selective amplification using blocking oligonucleotides

Description:
RELATED APPLICATIONS 
     The present application is a continuation of U.S. patent application Ser. No. 15/875,816, filed on Jan. 19, 2018, which claims priority under 35 U.S.C. § 119(e) to U.S. Provisional Application No. 62/453,163, filed on Feb. 1, 2017. The content of these related applications is expressly incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     The expression level of different genes can vary significantly in a biological sample. For examples, some broad categories of gene expression are: 1) “high expressers” which are comprised of 5-10 genes that dominate ˜20% of cellular mRNAs; 2) “intermediate expressers” that are comprised of 50-200 genes that occupy 40-60% of cellular mRNAs; and 3) “moderate expressers” that are comprised of 10,000-20,000 genes that occupy the rest of the cellular mRNA fraction. One challenge in molecular biology and molecular genetics is to capture this highly dynamic gene expression profile efficiently and accurately in order to distinguish different cell types and phenotypes in the sample. 
     In recent years, next generation sequencing (NGS) has provided a high throughput method in assessing gene expression profiles. During library preparation for NGS, a sample with heterogeneous cDNA species is amplified by PCR to obtain adequate sample amount and to attach NGS-compatible adapters. The sequencing process captures the number of reads for each gene from the PCR-amplified library sample to interpret the gene expression level. However, since different genes are expressed at a large range of levels, PCR amplification can skew the native gene expression. For example, a gene has 1 molecule of cDNA would require 40 cycles of PCR to achieve the same representative amount as a gene with 1000 molecules of cDNA in 30 cycles. In a heterogeneous cDNA sample, PCR is usually performed in excess cycles to adequately amplify low expressers; in those scenarios, the native gene expression profile is usually skewed by the dominating high expresser PCR products. A method to correct for such a bias in PCR product is Molecular Indexing; however, high expressers such as ribosomal protein mRNAs, mitochondrial mRNAs, or housekeeping genes often dominate the sequencing run with little contribution to the experimental interpretation. There is a need for selectively amplifying sequences of interest. 
     SUMMARY 
     Disclosed herein is a method of selective amplification. In some embodiments, the method comprises: providing a sample comprising a plurality of nucleic acid target molecules and one or more undesirable nucleic acid species; providing a plurality of oligonucleotide probes, wherein each of the plurality of oligonucleotide probes comprises a molecular label sequence and a binding region; contacting the plurality of oligonucleotide probes with the plurality of nucleic acid target molecules for hybridization; extending oligonucleotide probes that are hybridized to the plurality of nucleic acid target molecules to generate a plurality of extension products; providing a blocking oligonucleotide that specifically binds to at least one of the one or more undesirable nucleic acid species; and amplifying the plurality of extension products to generate a plurality of amplicons, whereby the amplification or the extension of the undesirable nucleic acid species is reduced by the blocking oligonucleotide. Also disclosed herein is a method of selective extension. In some embodiments, the method comprises: providing a sample comprising a plurality of nucleic acid target molecules and one or more undesirable nucleic acid species; providing a plurality of oligonucleotide probes, wherein each of the plurality of oligonucleotide probes comprises a molecular label sequence and a binding region; contacting the plurality of oligonucleotide probes with the plurality of nucleic acid target molecules for hybridization; providing a blocking oligonucleotide that specifically binds to at least one of the one or more undesirable nucleic acid species; and extending oligonucleotide probes that are hybridized to the plurality of nucleic acid target molecules to generate a plurality of extension products; whereby the extension of the undesirable nucleic acid species is reduced by the blocking oligonucleotide. 
     In the methods and compositions disclosed herein, the blocking oligonucleotide can be, for example, a locked nucleic acid (LNA), a peptide nucleic acid (PNA), a DNA, an LNA/PNA chimera, an LNA/DNA chimera, or a PNA/DNA chimera. In some embodiments, the methods comprise providing blocking oligonucleotides that specifically binds to two or more undesirable nucleic acid species in the sample. In some embodiments, the methods comprise providing blocking oligonucleotides that specifically binds to at least 10 undesirable nucleic acid species in the sample. In some embodiments, the methods comprise providing blocking oligonucleotides that specifically binds to at least 100 undesirable nucleic acid species in the sample. 
     The blocking oligonucleotide can have a Tm of at least 60° C., a Tm of at least 65° C., or a Tm of at least 70° C. In some embodiments, the blocking oligonucleotide is unable to function as a primer for a reverse transcriptase or a polymerase. In some embodiments, the amplification or the extension of the undesirable nucleic acid species is reduced by at least 50%. In some embodiments, the amplification or the extension of the undesirable nucleic acid species is reduced by at least 80%. In some embodiments, the amplification or the extension of the undesirable nucleic acid species is reduced by at least 90%. In some embodiments, the amplification or the extension of the undesirable nucleic acid species is reduced by at least 95%. In some embodiments, the amplification or the extension of the undesirable nucleic acid species is reduced by at least 99%. 
     In some embodiments, the blocking oligonucleotide is 10 nt to 50 nt long. In some embodiments, the blocking oligonucleotide is 20 nt to 30 nt long. In some embodiments, the blocking oligonucleotide is about 25 nt long. 
     In some embodiments, the one or more undesirable nucleic acid species amounts to about 50% of the nucleic acid content of the sample. In some embodiments, the one or more undesirable nucleic acid species amounts to about 60% of the nucleic acid content of the sample. In some embodiments, the one or more undesirable nucleic acid species amounts to about 70% of the nucleic acid content of the sample. In some embodiments, the one or more undesirable nucleic acid species amounts to about 80% of the nucleic acid content of the sample. 
     In some embodiments, the undesirable nucleic acid species is selected from the group consisting of rRNA, mtRNA, genomic DNA, intronic sequence, high abundance sequence, and any combination thereof. In some embodiments, the blocking oligonucleotides specifically bind to within 100 nt of the 3′ end of the one or more undesirable nucleic acid species. In some embodiments, the blocking oligonucleotides specifically bind to within 100 nt of the 5′ end of the one or more undesirable nucleic acid species. In some embodiments, the blocking oligonucleotides specifically bind to within 100 nt of the middle of the one or more undesirable nucleic acid species. 
     In some embodiments, the methods further comprise removing the hybridized complex formed between the blocking oligonucleotide and the undesirable nucleic acid species. In some embodiments, the removing comprises immobilizing the hybridized complex formed between the blocking oligonucleotide and the undesirable nucleic acid species on a solid support. In some embodiments, the blocking oligonucleotide comprises an affinity moiety. In some embodiments, solid support comprises a binding partner of the affinity moiety. In some embodiments, the affinity moiety is a functional group selected from the group consisting of biotin, streptavidin, heparin, an aptamer, a click-chemistry moiety, digoxigenin, primary amine(s), carboxyl(s), hydroxyl(s), aldehyde(s), ketone(s), and any combination thereof. In some embodiments, the affinity moiety is biotin. In some embodiments, the solid support comprises streptavidin. 
     In some embodiments, the amplifying comprises PCR amplification of the plurality of extension products. In some embodiments, each of the plurality of oligonucleotide probes comprises a cell label sequence, a sample label sequence, a location label sequence, a binding site for a universal primer, or any combination thereof. In some embodiments, the plurality of oligonucleotide probes comprises at least 100 different molecular label sequences. In some embodiments, the plurality of oligonucleotide probes comprises at least 1,000 different molecular label sequences. In some embodiments, the plurality of oligonucleotide probes comprises at least 10,000 different molecular label sequences. In some embodiments, the plurality of oligonucleotide probes comprises the same cell label sequence. In some embodiments, the plurality of amplicons comprises a cDNA library. In some embodiments, the sample comprises a single cell, a plurality of cells, a tissue sample, or any combination thereof. In some embodiments, the sample is a single cell. 
     In some embodiments, the methods further comprise sequencing the plurality of amplicons. In some embodiments, the undesirable nucleic acid species represents less than 50% of the plurality of amplicons. In some embodiments, the undesirable nucleic acid species represents less than 20% of the plurality of amplicons. In some embodiments, the undesirable nucleic acid species represents less than 10% of the plurality of amplicons. In some embodiments, the undesirable nucleic acid species represents less than 5% of the plurality of amplicons. In some embodiments, the plurality of oligonucleotide probes is immobilized on a substrate. In some embodiments, the substrate is a particle. In some embodiments, the substrate is a bead. In some embodiments, the plurality of nucleic acid target molecules comprises mRNA target molecules. In some embodiments, the binding region comprises poly-dT sequence. 
     Also disclosed herein is a kit for selective amplification of a plurality of nucleic acid target molecules in a sample. In some embodiments, the kit comprises: a plurality of oligonucleotide probes, wherein each of the plurality of oligonucleotide probes comprises a molecular label sequence and a binding region; and a plurality of blocking oligonucleotides that specifically binds to a plurality of undesirable nucleic acid species in the sample, wherein each blocking oligonucleotide probe is unable to function as a primer for a reverse transcriptase or a polymerase. 
     In some embodiments, the blocking oligonucleotide is a locked nucleic acid (LNA), a peptide nucleic acid (PNA), a DNA, an LNA/PNA chimera, an LNA/DNA chimera, or a PNA/DNA chimera. In some embodiments, the kit comprises blocking oligonucleotides that specifically binds to two or more undesirable nucleic acid species. In some embodiments, the kit comprises blocking oligonucleotides that specifically binds to at least 10 undesirable nucleic acid species. In some embodiments, the kit comprises blocking oligonucleotides that specifically binds to at least 100 undesirable nucleic acid species. 
     In some embodiments, the blocking oligonucleotide is 10 nt to 50 nt long. In some embodiments, the blocking oligonucleotide is 20 nt to 30 nt long. In some embodiments, the blocking oligonucleotide is about 25 nt long. In some embodiments, the undesirable nucleic acid species is selected from the group consisting of rRNA, mtRNA, genomic DNA, intronic sequence, high abundance sequence, and any combination thereof. 
     In some embodiments, the blocking oligonucleotides specifically bind to within 100 nt of the 3′ end of the undesirable nucleic acid species. In some embodiments, the blocking oligonucleotides specifically bind to within 100 nt of the 5′ end of the undesirable nucleic acid species. In some embodiments, the blocking oligonucleotides specifically bind to within 100 nt of the middle of the undesirable nucleic acid species. In some embodiments, the blocking oligonucleotide comprises an affinity moiety. 
     In some embodiments, each of the plurality of oligonucleotide probes comprises a cell label sequence, a sample label sequence, a location label sequence, a binding site for a universal primer, or any combination thereof. In some embodiments, the binding region comprises poly-dT. In some embodiments, the plurality of oligonucleotide probes is immobilized on a substrate. In some embodiments, the substrate is a particle, for example a bead. 
     In some embodiments, the kit further comprises an enzyme. The enzyme can be, for example, a reverse transcriptase, a polymerase, a ligase, a nuclease, and any combination thereof. In some embodiments, each blocking oligonucleotide probe has a Tm of at least 60° C. In some embodiments, the plurality of oligonucleotide probes comprises at least 100 different molecular label sequences. In some embodiments, the plurality of oligonucleotide probes comprises at least 1,000 different molecular label sequences. In some embodiments, the plurality of oligonucleotide probes comprises at least 10,000 different molecular label sequences. In some embodiments, the plurality of oligonucleotide probes comprises the same cell label sequence. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a schematic illustration of an exemplary method for selective amplification using blocking oligonucleotides that specifically bind to undesirable nucleic acid species. 
     
    
    
     DETAILED DESCRIPTION 
     Definitions 
     Unless otherwise defined, all technical terms used herein have the same meaning as commonly understood by one of ordinary skill in the art in the field to which this disclosure belongs. As used in this specification and the appended claims, the singular forms “a,” “an,” and “the” include plural references unless the context clearly dictates otherwise. Any reference to “or” herein is intended to encompass “and/or” unless otherwise stated. 
     As used herein the term “associated” or “associated with” can mean that two or more species are identifiable as being co-located at a point in time. An association can mean that two or more species are or were within a similar container. An association can be an informatics association, where for example digital information regarding two or more species is stored and can be used to determine that one or more of the species were co-located at a point in time. An association can also be a physical association. In some instances two or more associated species are “tethered”, “attached”, or “immobilized” to one another or to a common solid or semisolid surface. An association may refer to covalent or non-covalent means for attaching labels to solid or semi-solid supports such as beads. An association may comprise hybridization between a target and a label. 
     As used herein, the term “complementary” can refer to the capacity for precise pairing between two nucleotides. For example, if a nucleotide at a given position of a nucleic acid is capable of hydrogen bonding with a nucleotide of another nucleic acid, then the two nucleic acids are considered to be complementary to one another at that position. Complementarity between two single-stranded nucleic acid molecules may be “partial,” in which only some of the nucleotides bind, or it may be complete when total complementarity exists between the single-stranded molecules. A first nucleotide sequence can be said to be the “complement” of a second sequence if the first nucleotide sequence is complementary to the second nucleotide sequence. A first nucleotide sequence can be said to be the “reverse complement” of a second sequence, if the first nucleotide sequence is complementary to a sequence that is the reverse (i.e., the order of the nucleotides is reversed) of the second sequence. As used herein, the terms “complement”, “complementary”, and “reverse complement” can be used interchangeably. It is understood from the disclosure that if a molecule can hybridize to another molecule it may be the complement of the molecule that is hybridizing. 
     As used herein, the term “digital counting” can refer to a method for estimating a number of target molecules in a sample. Digital counting can include the step of determining a number of unique labels that have been associated with targets in a sample. This stochastic methodology transforms the problem of counting molecules from one of locating and identifying identical molecules to a series of yes/no digital questions regarding detection of a set of predefined labels. 
     As used herein, the term “label” or “labels” can refer to nucleic acid codes associated with a target within a sample. A label can be, for example, a nucleic acid label. A label can be an entirely or partially amplifiable label. A label can be entirely or partially sequencable label. A label can be a portion of a native nucleic acid that is identifiable as distinct. A label can be a known sequence. A label can comprise a junction of nucleic acid sequences, for example a junction of a native and non-native sequence. As used herein, the term “label” can be used interchangeably with the terms, “index”, “tag,” or “label-tag.” Labels can convey information. For example, in various embodiments, labels can be used to determine an identity of a sample, a source of a sample, an identity of a cell, and/or a target. 
     As used herein, a “nucleic acid” can generally refer to a polynucleotide sequence, or fragment thereof. A nucleic acid can comprise nucleotides. A nucleic acid can be exogenous or endogenous to a cell. A nucleic acid can exist in a cell-free environment. A nucleic acid can be a gene or fragment thereof. A nucleic acid can be DNA. A nucleic acid can be RNA. A nucleic acid can comprise one or more analogs (e.g. altered backgone, sugar, or nucleobase). Some non-limiting examples of analogs include: 5-bromouracil, peptide nucleic acid, xeno nucleic acid, morpholinos, locked nucleic acids, glycol nucleic acids, threose nucleic acids, dideoxynucleotides, cordycepin, 7-deaza-GTP, florophores (e.g. rhodamine or flurescein linked to the sugar), thiol containing nucleotides, biotin linked nucleotides, fluorescent base analogs, CpG islands, methyl-7-guanosine, methylated nucleotides, inosine, thiouridine, pseudourdine, dihydrouridine, queuosine, and wyosine. “Nucleic acid”, “polynucleotide, “target polynucleotide”, and “target nucleic acid” can be used interchangeably. 
     A nucleic acid can comprise one or more modifications (e.g., a base modification, a backbone modification), to provide the nucleic acid with a new or enhanced feature (e.g., improved stability). A nucleic acid can comprise a nucleic acid affinity tag. A nucleoside can be a base-sugar combination. The base portion of the nucleoside can be a heterocyclic base. The two most common classes of such heterocyclic bases are the purines and the pyrimidines. Nucleotides can be nucleosides that further include a phosphate group covalently linked to the sugar portion of the nucleoside. For those nucleosides that include a pentofuranosyl sugar, the phosphate group can be linked to the 2′, the 3′, or the 5′ hydroxyl moiety of the sugar. In forming nucleic acids, the phosphate groups can covalently link adjacent nucleosides to one another to form a linear polymeric compound. In turn, the respective ends of this linear polymeric compound can be further joined to form a circular compound; however, linear compounds are generally suitable. In addition, linear compounds may have internal nucleotide base complementarity and may therefore fold in a manner as to produce a fully or partially double-stranded compound. Within nucleic acids, the phosphate groups can commonly be referred to as forming the internucleoside backbone of the nucleic acid. The linkage or backbone of the nucleic acid can be a 3′ to 5′ phosphodiester linkage. 
     A nucleic acid can comprise a modified backbone and/or modified internucleoside linkages. Modified backbones can include those that retain a phosphorus atom in the backbone and those that do not have a phosphorus atom in the backbone. Suitable modified nucleic acid backbones containing a phosphorus atom therein can include, for example, phosphorothioates, chiral phosphorothioates, phosphorodithioates, phosphotriesters, aminoalkylphosphotriesters, methyl and other alkyl phosphonates such as 3′-alkylene phosphonates, 5′-alkylene phosphonates, chiral phosphonates, phosphinates, phosphoramidates including 3′-amino phosphoramidate and aminoalkylphosphoramidates, phosphorodiamidates, thionophosphoramidates, thionoalkylphosphonates, thionoalkylphosphotriesters, selenophosphates, and boranophosphates having normal 3′-5′ linkages, 2′-5′ linked analogs, and those having inverted polarity wherein one or more internucleotide linkages is a 3′ to 3′, a 5′ to 5′ or a 2′ to 2′ linkage. 
     A nucleic acid can comprise polynucleotide backbones that are formed by short chain alkyl or cycloalkyl internucleoside linkages, mixed heteroatom and alkyl or cycloalkyl internucleoside linkages, or one or more short chain heteroatomic or heterocyclic internucleoside linkages. These can include those having morpholino linkages (formed in part from the sugar portion of a nucleoside); siloxane backbones; sulfide, sulfoxide and sulfone backbones; formacetyl and thioformacetyl backbones; methylene formacetyl and thioformacetyl backbones; riboacetyl backbones; alkene containing backbones; sulfamate backbones; methyleneimino and methylenehydrazino backbones; sulfonate and sulfonamide backbones; amide backbones; and others having mixed N, O, S and CH 2  component parts. 
     A nucleic acid can comprise a nucleic acid mimetic. The term “mimetic” can be intended to include polynucleotides wherein only the furanose ring or both the furanose ring and the internucleotide linkage are replaced with non-furanose groups, replacement of only the furanose ring can also be referred as being a sugar surrogate. The heterocyclic base moiety or a modified heterocyclic base moiety can be maintained for hybridization with an appropriate target nucleic acid. One such nucleic acid can be a peptide nucleic acid (PNA). In a PNA, the sugar-backbone of a polynucleotide can be replaced with an amide containing backbone, in particular an aminoethylglycine backbone. The nucleotides can be retained and are bound directly or indirectly to aza nitrogen atoms of the amide portion of the backbone. The backbone in PNA compounds can comprise two or more linked aminoethylglycine units which gives PNA an amide containing backbone. The heterocyclic base moieties can be bound directly or indirectly to aza nitrogen atoms of the amide portion of the backbone. 
     A nucleic acid can comprise a morpholino backbone structure. For example, a nucleic acid can comprise a 6-membered morpholino ring in place of a ribose ring. In some of these embodiments, a phosphorodiamidate or other non-phosphodiester internucleoside linkage can replace a phosphodiester linkage. 
     A nucleic acid can comprise linked morpholino units (i.e. morpholino nucleic acid) having heterocyclic bases attached to the morpholino ring. Linking groups can link the morpholino monomeric units in a morpholino nucleic acid. Non-ionic morpholino-based oligomeric compounds can have less undesired interactions with cellular proteins. Morpholino-based polynucleotides can be nonionic mimics of nucleic acids. A variety of compounds within the morpholino class can be joined using different linking groups. A further class of polynucleotide mimetic can be referred to as cyclohexenyl nucleic acids (CeNA). The furanose ring normally present in a nucleic acid molecule can be replaced with a cyclohexenyl ring. CeNA DMT protected phosphoramidite monomers can be prepared and used for oligomeric compound synthesis using phosphoramidite chemistry. The incorporation of CeNA monomers into a nucleic acid chain can increase the stability of a DNA/RNA hybrid. CeNA oligoadenylates can form complexes with nucleic acid complements with similar stability to the native complexes. A further modification can include Locked Nucleic Acids (LNAs) in which the 2′-hydroxyl group is linked to the 4′ carbon atom of the sugar ring thereby forming a 2′-C,4′-C-oxymethylene linkage thereby forming a bicyclic sugar moiety. The linkage can be a methylene (—CH2-), group bridging the 2′ oxygen atom and the 4′ carbon atom wherein n is 1 or 2. LNA and LNA analogs can display very high duplex thermal stabilities with complementary nucleic acid (Tm=+3 to +10° C.), stability towards 3′-exonucleolytic degradation and good solubility properties. 
     A nucleic acid may also include nucleobase (often referred to simply as “base”) modifications or substitutions. As used herein, “unmodified” or “natural” nucleobases can include the purine bases, (e.g. adenine (A) and guanine (G)), and the pyrimidine bases, (e.g. thymine (T), cytosine (C) and uracil (U)). Modified nucleobases can include other synthetic and natural nucleobases such as 5-methylcytosine (5-me-C), 5-hydroxymethyl cytosine, xanthine, hypoxanthine, 2-aminoadenine, 6-methyl and other alkyl derivatives of adenine and guanine, 2-propyl and other alkyl derivatives of adenine and guanine, 2-thiouracil, 2-thiothymine and 2-thiocytosine, 5-halouracil and cytosine, 5-propynyl (—C═C—CH3) uracil and cytosine and other alkynyl derivatives of pyrimidine bases, 6-azo uracil, cytosine and thymine, 5-uracil (pseudouracil), 4-thiouracil, 8-halo, 8-amino, 8-thiol, 8-thioalkyl, 8-hydroxyl and other 8-substituted adenines and guanines, 5-halo particularly 5-bromo, 5-trifluoromethyl and other 5-substituted uracils and cytosines, 7-methylguanine and 7-methyladenine, 2-F-adenine, 2-aminoadenine, 8-azaguanine and 8-azaadenine, 7-deazaguanine and 7-deazaadenine and 3-deazaguanine and 3-deazaadenine. Modified nucleobases can include tricyclic pyrimidines such as phenoxazine cytidine(1H-pyrimido(5,4-b)(1,4)benzoxazin-2(3H)-one), phenothiazine cytidine (1H-pyrimido(5,4-b)(1,4)benzothiazin-2(3H)-one), G-clamps such as a substituted phenoxazine cytidine (e.g. 9-(2-aminoethoxy)-H-pyrimido(5,4-(b) (1,4)benzoxazin-2(3H)-one), carbazole cytidine (2H-pyrimido(4,5-b)indol-2-one), pyridoindole cytidine (Hpyrido(3′,′:4,5)pyrrolo[2,3-d]pyrimidin-2-one). 
     As used herein, the term “sample” can refer to a composition comprising targets. Suitable samples for analysis by the disclosed methods, devices, and systems include cells, single cells, tissues, organs, or organisms. 
     As used herein, the term “sampling device” or “device” can refer to a device which may take a section of a sample and/or place the section on a substrate. A sample device can refer to, for example, an fluorescence activated cell sorting (FACS) machine, a cell sorter machine, a biopsy needle, a biopsy device, a tissue sectioning device, a microfluidic device, a blade grid, and/or a microtome. 
     As used herein, the term “solid support” can refer to discrete solid or semi-solid surfaces to which a plurality of stochastic barcodes may be attached. A solid support may encompass any type of solid, porous, or hollow sphere, ball, bearing, cylinder, or other similar configuration composed of plastic, ceramic, metal, or polymeric material (e.g., hydrogel) onto which a nucleic acid may be immobilized (e.g., covalently or non-covalently). A solid support may comprise a discrete particle that may be spherical (e.g., microspheres) or have a non-spherical or irregular shape, such as cubic, cuboid, pyramidal, cylindrical, conical, oblong, or disc-shaped, and the like. A plurality of solid supports spaced in an array may not comprise a substrate. A solid support may be used interchangeably with the term “bead.” As used herein, “solid support” and “substrate” can be used interchangeably. 
     As used herein, the term “stochastic barcode” refers to a polynucleotide sequence comprising labels of the present disclosure. A stochastic barcode can be a polynucleotide sequence that can be used for stochastic barcoding. Stochastic barcodes can be used to quantify targets within a sample. Stochastic barcodes can be used to control for errors which may occur after a label is associated with a target. For example, a stochastic barcode can be used to assess amplification or sequencing errors. A stochastic barcode associated with a target can be called a stochastic barcode-target or stochastic barcode-tag-target. 
     As used herein, the term “stochastic barcoding” refers to the random labeling (e.g., barcoding) of nucleic acids. Stochastic barcoding can utilize a recursive Poisson strategy to associate and quantify labels associated with targets. As used herein, the term “stochastic barcoding” can be used interchangeably with “stochastic labeling.” 
     As used here, the term “target” can refer to a composition which can be associated with a stochastic barcode. Exemplary suitable targets for analysis by the disclosed methods, devices, and systems include oligonucleotides, DNA, RNA, mRNA, microRNA, tRNA, and the like. Targets can be single or double stranded. In some embodiments targets can be proteins, polypeptides or peptides. In some embodiments targets are lipids. As used herein, “target” can be used interchangeably with “species”. 
     The term “reverse transcriptases” can refer to a group of enzymes having reverse transcriptase activity (i.e., that catalyze synthesis of DNA from an RNA template). In general, such enzymes include, but are not limited to, retroviral reverse transcriptase, retrotransposon reverse transcriptase, retroplasmid reverse transcriptases, retron reverse transcriptases, bacterial reverse transcriptases, group II intron-derived reverse transcriptase, and mutants, variants or derivatives thereof. Non-retroviral reverse transcriptases include non-LTR retrotransposon reverse transcriptases, retroplasmid reverse transcriptases, retron reverse transciptases, and group II intron reverse transcriptases. Examples of group II intron reverse transcriptases include the Lactococc s  lactis  Ll.LtrB intron reverse transcriptase, the  Thermosynechococcus elongatus  TeI4c intron reverse transcriptase, or the  Geobacillus stearothermophilus  GsI-IIC intron reverse transcriptase. Other classes of reverse transcriptases can include many classes of non-retroviral reverse transcriptases (i.e., retrons, group II introns, and diversity-generating retroelements among others). 
     Methods of Selective Amplification and/or Extension 
     Some embodiments disclosed herein provide methods of selective amplification and/or extension of a plurality of nucleic acid target molecules in a sample. The methods and compositions can, for example, reduce the amplification and/or extension of undesirable nucleic acid species in the sample, allow selective removal of undesirable nucleic acid species of the sample, or both. 
     For example, a sample can comprise a plurality of nucleic acid target molecules, and one or more undesirable nucleic acid species. In some embodiments, the method can significantly reduce the amplification, the extension, or both of the one or more undesirable nucleic acid species as compared to the plurality of nucleic acid target molecules in the sample. For example, the amplification and/or extension of the one or more undesirable nucleic acid species can be reduced by at least 10%, at least 20%, at least 30%, at least 40%, at least 50%, at least 60%, at least 70%, at least 80%, at least 90%, at least 95%, at least 99% or more in comparison to the amplification and/or extension of at least one of the nucleic acid target molecules in the sample, or the average amplification and/or extension of the one or more of the nucleic acid target molecules in the sample. In some embodiments, the methods disclosed herein can significantly reduce the amplification and/or extension of the one or more undesirable nucleic acid species as compared to the nucleic acid target molecules without significantly affecting the amplification and/or extension of the nucleic acid target molecules in the sample. 
     As used herein, a “nucleic acid species” refers to polynucleotides (for example, single-stranded polynucleotides) that are the same or substantially the same in sequence, or complement of one another, or are capable of hybridize to one another, or are transcripts from the same genetic locus, or encode the same protein or fragment thereof. In some embodiments, members of a nucleic acid species are at least 80%, at least 90%, at least 95%, at least 98%, at least 99%, or 100% homologous to one another, or complement thereof. In some embodiments, members of a species can hybridize to one another under high stringent hybridization conditions. In some embodiments, members of a species can hybridize to one another under moderate stringent hybridization conditions. In some embodiments, members of a species can hybridize to one another under low stringent hybridization conditions. In some embodiments, members of a species are transcripts from the same genetic locus and the transcripts can be of the same or different length. The species is, in some embodiments, genomic DNA, ribosomal RNA (rRNA), mitochondrial DNA (mtDNA), cDNA, mRNA, or a combination thereof. 
     In some embodiments, the methods and compositions disclosed herein can reduce the amplification and/or extension of one or more undesirable nucleic acid species in a sample. For example, the methods and compositions disclosed herein can reduce the amplification and/or extension of at least 1, at least 2, at least 3, at least 4, at least 5, at least 10, at least 20, at least 50, at least 100, at least 200, at least 500, at least 1,000, or more, undesirable nucleic acid species in the sample. In some embodiments, the methods and compositions disclosed herein can reduce the amplification and/or extension by at least 10%, at least 20%, at least 30%, at least 40%, at least 50%, at least 60%, at least 70%, at least 80%, at least 90%, at least 95%, at least 99%, or 100% of each of the one or more undesirable nucleic acid species in the sample. In some embodiments, the methods and compositions disclosed herein abolish the amplification and/or extension of each of the one or more undesirable nucleic acid species in the sample. In some embodiments, the methods and compositions disclosed herein can reduce amplification and/or extension by at least 10%, at least 20%, at least 30%, at least 40%, at least 50%, at least 60%, at least 70%, at least 80%, at least 90%, at least 95%, at least 99%, or 100% of at least one of the one or more undesirable nucleic acid species. In some embodiments, the methods and compositions disclosed herein abolish amplification and/or extension of at least one of the one or more undesirable nucleic acid species. In some embodiments, the methods and compositions disclosed herein reduce the amplification and/or extension of the total of undesirable nucleic acid species. 
     In some embodiments, the methods and compositions disclosed herein can reduce the amplification and/or extension of one or more undesirable nucleic acid species without significantly reducing amplification and/or extension of the nucleic acid target molecules in the same sample. For example, in some embodiments, the methods and compositions disclosed herein can reduce the amplification and/or extension by at least 10%, at least 20%, at least 30%, at least 40%, at least 50%, at least 60%, at least 70%, at least 80%, at least 90%, at least 95%, or 100% for each of the one or more undesirable nucleic acid species without significantly reducing amplification and/or extension of the nucleic acid target molecules. In some embodiments, the methods and compositions disclosed herein can reduce the amplification and/or extension by at least 10%, at least 20%, at least 30%, at least 40%, at least 50%, at least 60%, at least 70%, at least 80%, at least 90%, at least 95%, or 100% of the total of undesirable nucleic acid species without significantly reducing amplification and/or extension of the nucleic acid target molecules. In some embodiments, the methods and compositions disclosed herein can reduce the amplification and/or extension of one or more undesirable nucleic acid species while keeping at least at least 50%, at least 60%, at least 70%, at least 80%, at least 90%, at least 95%, or 100% of the amplification and/or extension of each of the nucleic acid target molecules. In some embodiments, the methods and compositions disclosed herein can reduce the amplification and/or extension of one or more undesirable nucleic acid species while keeping at least at least 50%, at least 60%, at least 70%, at least 80%, at least 90%, at least 95%, or 100% of the amplification and/or extension of at least one of the nucleic acid target molecules. In some embodiments, the methods and compositions disclosed herein can reduce the amplification and/or extension of one or more undesirable nucleic acid species while keeping at least at least 50%, at least 60%, at least 70%, at least 80%, at least 90%, at least 95%, or 100% of the amplification and/or extension of the total of the nucleic acid target molecules. 
     As shown in  FIG. 1 , a sample comprises a whole transcriptome amplification (WTA) product  100  contains an undesirable nucleic acid species  105  and a nucleic acid target molecule  110 . During an amplification reaction  120 , an LNA/PNA blocking oligonucleotide  125  that specifically binds to the undesirable nucleic acid species  105  (binding shown at the 3′ end of  105  as a non-limiting example) is provided, which reduces or inhibits the amplification of the undesirable nucleic acid species  105 . On the other hand, a copy  130  of the nucleic acid target molecule  110  is synthesized. In some embodiments, the amplification reaction  120  can be used in ending amplification step of WTA after end selection. In some embodiments, the hybridization complex between the undesirable nucleic acid species  105  and the blocking oligonucleotide  125  may be removed, for example, by immobilizing to a solid support. Completion  140  of the amplification reaction results in a library  150  comprising multiple copies  145  of the nucleic acid target molecule  110 . 
     Nucleic Acid Target Molecules 
     In some embodiments, the methods disclosed herein comprise providing a sample comprising a plurality of nucleic acid target molecules. It would be appreciated by one of skill in the art that the plurality of nucleic acid target molecules can comprise a variety of nucleic acid target molecules. For example, the nucleic acid target molecules can comprise DNA molecules, RNA molecules, genomic DNA molecules, cDNA molecules, mRNA molecules, rRNA molecules, mtDNA, siRNA molecules, or any combination thereof. The nucleic acid target molecule can be double-stranded or single-stranded. In some embodiments, the plurality of nucleic acid target molecules can comprise polyA RNA molecules. In some embodiments, the plurality of nucleic acid target molecules comprise at least 100, at least 1,000, at least 10,000, at least 20,000, at least 30,000, at least 40,000, at least 50,000, at least 100,000, at least 1,000,000, or more nucleic acid species. In some embodiments, the plurality of nucleic acid target molecules can be from a sample, such as a single cell, a tissue, or a plurality of cells. In some embodiments, the plurality of nucleic acid target molecules can be pooled from a plurality of samples, such as a plurality of single cells or samples from different subjects (e.g., patients). 
     In some embodiments, the sample can comprise one or more undesirable nucleic acid species. As used herein, an “undesirable nucleic acid species” refers to a nucleic acid species that is present, e.g., in high amount, in a sample, for example the nucleic acid species representing 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 20%, 30%, 40%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, or more, or a range between any two of these values of the nucleic acid content in the sample. In some embodiments, the sample can comprise at least 1, at least 2, at least 3, at least 4, at least 5, at least 10, at least 20, at least 50, at least 100, at least 200, at least 500, at least 1,000, or more, undesirable nucleic acid species. In some embodiments, the total of all the undesirable nucleic acid species represent at least 10%, at least 20%, at least 30%, at least 40%, at least 50%, at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, at least 80%, or more of the nucleic acid content in the sample. In some embodiments, undesirable nucleic acid species can comprise polynucleotides encoding one or more ribosomal proteins. In some embodiments, undesirable nucleic acid species comprise rRNA. In some embodiments, undesirable nucleic acid species can comprise polynucleotides encoding one or more mitochondrial proteins. In some embodiments, undesirable nucleic acid species comprise mtDNA. In some embodiments, undesirable nucleic acid species can comprise polynucleotides encoding one or more housekeeping proteins. In some embodiments, undesirable nucleic acid species can comprise mRNA, rRNA, mtRNA, genomic DNA, intronic sequence, high abundance sequence, and any combination thereof. 
     In some embodiments, the plurality of nucleic acid target molecules comprises an unnormalized nucleic acid library, a partially normalized nucleic acid library, or a nucleic acid library that has been normalized by other methods, such as a cDNA library, a genomic DNA library, or the like. In some embodiments, the plurality of nucleic acid target molecules can comprise a pooled unnormalized nucleic acid library, such as a pooled unnormalized nucleic acid library constructed from a plurality of unnormalized nucleic acid libraries each representing a single cell. In some embodiments, the unnormalized nucleic acid library is a cDNA library. In some embodiments, the unnormalized nucleic acid library is a genomic library. In some embodiments, the unnormalized nucleic acid library is a single-cell nucleic acid library. 
     Blocking Oligonucleotides 
     In some embodiments, the methods disclosed herein comprise providing a blocking oligonucleotide that specifically binds to at least one of the one or more undesirable nucleic acid species. The blocking oligonucleotides can be provided at any point during the methods disclosed herein so that they can reduce the amplification and/or extension of the undesirable nucleic acid species. For example, the blocking oligonucleotides can be provided before, during or after the extension step, before or during the amplification step, before, during or after providing a plurality of oligonucleotide steps, before, during or after contacting the plurality of oligonucleotide probes with the plurality of nucleic acid target molecules for hybridization, or any combination thereof. A “blocking oligonucleotide” as used herein refers to a nucleic acid molecule that can specifically bind to at least one of the one or more undesirable nucleic acid species, whereby the specifically binding between the blocking oligonucleotide and the one or more undesirable nucleic acid species can reduce the amplification or extension (e.g., reverse transcription) of the one or more undesirable nucleic acid species. For example, the blocking oligonucleotide can comprise a nucleic acid sequence capable of hybridizing with one or more undesirable nucleic acid species. In some embodiments, a plurality of blocking oligonucleotides can be provided. The plurality of blocking oligonucleotides can specifically bind to at least 1, at least 2, at least 5, at least 10, at least 100, at least 1,000 or more of the one or more undesirable nucleic acid species. The location at which a blocking oligonucleotide specifically binds to an undesirable nucleic acid species can vary. For example, blocking oligonucleotide can specifically binds to a sequence close to the 5′ end of the undesirable nucleic acid species. In some embodiments, the blocking oligonucleotide can specifically bind to within 10 nt, 20 nt, 30 nt, 40 nt, 50 nt, 100 nt, 200 nt, 300 nt, 400 nt, 500 nt, or 1,000 nt of the 5′ end of at least one of the one or more undesirable nucleic acid species. In some embodiments, blocking oligonucleotide can specifically binds to a sequence close to the 3′ end of the undesirable nucleic acid species. For example, the blocking oligonucleotide can specifically bind to within 10 nt, 20 nt, 30 nt, 40 nt, 50 nt, 100 nt, 200 nt, 300 nt, 400 nt, 500 nt, 1,000 nt of the 3′ end of at least one of the one or more undesirable nucleic acid species. As another example, blocking oligonucleotide can specifically binds to a sequence in the middle portion of the undesirable nucleic acid species. In some embodiments, the blocking oligonucleotide can specifically bind to within 10 nt, 20 nt, 30 nt, 40 nt, 50 nt, 100 nt, 200 nt, 300 nt, 400 nt, 500 nt, 1,000 nt from the middle point of at least one of the one or more undesirable nucleic acid species. 
     In some embodiments, the specifically binding between the blocking oligonucleotide and the undesirable nucleic acid species can reduce the amplification and/or extension of the undesirable nucleic acid species by at least 10%, at least 20%, at least 30%, at least 40%, at least 50%, at least 60%, at least 70%, at least 80%, at least 90%, at least 95%, at least 98%, at least 99%, or more. 
     It is contemplated that the blocking oligonucleotide may reduce the amplification and/or extension of the undesirable nucleic acid species by, for example, forming a hybridization complex with the undesirable nucleic acid species having a high melting temperature (T m ), by not being able to function as a primer for a reverse transcriptase or a polymerase, a combination thereof, etc. In some embodiments, the blocking oligonucleotide can have a T m  that is, is about, is at least, 50° C., 60° C., 65° C., 70° C., 75° C., 80° C., 85° C., 90° C., 95° C., or a range between any two of these values. In some embodiments, the blocking oligonucleotide can reduce the amplification and/or extension of the undesirable nucleic acid species by competing with the amplification and/or extension primers for hybridization with the undesirable nucleic acid species. 
     The blocking oligonucleotide can, in some embodiments, comprise one or more non-natural nucleotides. Non-natural nucleotides can be, for example, photolabile or triggerable nucleotides. Examples of non-natural nucleotides can include, but are not limited to, peptide nucleic acid (PNA), morpholino and locked nucleic acid (LNA), as well as glycol nucleic acid (GNA) and threose nucleic acid (TNA). In some embodiments, the blocking oligonucleotide is a chimeric oligonucleotide, such as an LNA/PNA/DNA chimera, an LNA/DNA chimera, a PNA/DNA chimera, a GNA/DNA chimera, a TNA/DNA chimera, or a combination thereof. 
     The melting temperature (T m ) of a blocking oligonucleotide can be modified, in some embodiments, by adjusting the length of the blocking oligonucleotide. For example, a blocking oligonucleotide can have a length that is, is about, is less than, is more than, 10 nt, 15 nt, 20 nt, 25 nt, 30 nt, 35 nt, 40 nt, 45 nt, 50 nt, 60 nt, 70 nt, 80 nt, 90 nt, 100 nt, 200 nt, or a range between any two of the above values. 
     In some embodiments, the T m  of a blocking oligonucleotide is modified by the number of DNA residues in the blocking oligonucleotide that comprises an LNA/DNA chimera or a PNA/DNA chimera. For example, a blocking oligonucleotide that comprises an LNA/DNA chimera or a PNA/DNA chimera can have a percentage of DNA residues that is, is about, is less than, is more than, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 60%, 70%, 80%, 90%, or a range between any two of the above values. 
     In some embodiments, a blocking oligonucleotide can be designed to be incapable of functioning as a primer or probe for an amplification and/or extension reaction. For example, the blocking oligonucleotide may be incapable of function as a primer for a reverse transcriptase or a polymerase. For example, a blocking oligonucleotide that comprises an LNA/DNA chimera or a PNA/DNA chimera can be designed to have a certain percentage of LNA or PNA residues, or to have LNA or PNA residues on certain locations, such as close to or at the 3′ end, 5′ end, or in the middle portion of the oligonucleotide. In some embodiments, a blocking oligonucleotide that comprises an LNA/DNA chimera or a PNA/DNA chimera can have a percentage of LNA or PNA residues that is, is about, is less than, is more than, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 60%, 70%, 80%, 90%, or a range between any two of the above values. 
     In some embodiments, the methods disclosed herein can comprise removing the hybridized complex formed between the blocking oligonucleotide and the undesirable nucleic acid species. For example, the blocking oligonucleotides can comprise an affinity moiety. The affinity moiety can be a functional group selected from the group consisting of biotin, streptavidin, heparin, an aptamer, a click-chemistry moiety, digoxigenin, primary amine(s), carboxyl(s), hydroxyl(s), aldehyde(s), ketone(s), and any combination thereof. In some embodiments, the affinity moiety is biotin. In some embodiments, the blocking oligonucleotide can be immobilized to a solid support having a binding partner for the affinity moiety through the affinity moiety. In some embodiments, the binding partner is streptavidin. 
     Oligonucleotide Probes 
     In some embodiments, the methods disclosed herein comprise providing a plurality of oligonucleotide probes, wherein each of the plurality of oligonucleotide probes comprises a molecular label sequence and a binding region. In some embodiments, the methods disclosed herein comprise contacting the plurality of oligonucleotide probes with the plurality of nucleic acid target molecules for hybridization. The oligonucleotide probes can comprise a binding region that hybridizes to one or more of the plurality of nucleic acid target molecules and one or more of the undesirable nucleic acid species. In some embodiments, the binding region can be target specific. For example, the binding region is configured to bind specific sequence(s). In some embodiments, the binding region can be target nonspecific. In some embodiments, the binding region comprises or consists of poly-dT sequence. In some embodiments, the oligonucleotide probes can comprise a stochastic barcode. In some embodiments, the oligonucleotide probes can comprise a molecular label sequence, a cell label sequence, a sample label sequence, a location label sequence, a binding site for a universal primer, or any combination thereof. 
     It is contemplated that the methods and compositions disclosed herein can be used in conjunction of molecular label sequences, for example, oligonucleotide probes that comprise molecular label sequences. Accordingly, in some embodiments, the species of nucleic acid molecules as disclosed herein can include polynucleotides in the plurality of nucleic acid molecules that are the same or the complement of one another, or are capable of hybridize to one another, or are transcripts from the same genetic locus, or encode the same protein or fragment thereof, etc., but that are associated with different molecular label sequences. It would be appreciated that molecular label sequences can be used to identify occurrences of a nucleic acid species. 
     A molecular label sequence can comprise a nucleic acid sequence that provides identifying information for the specific nucleic acid. A molecular label sequence can comprise a nucleic acid sequence that provides a counter for the specific occurrence of the target nucleic acid. A molecular label sequence can be, for example, 1, 2, 3, 4, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, or more, or a range between any of these values, nucleotides in length. A molecular label sequence can be, for example, be at most about 300, 200, 100, 90, 80, 70, 60, 50, 40, 30, 20, 15, 12, 10, 9, 8, 7, 6, 5, 4 or fewer nucleotides in length. 
     It would be appreciated that in some embodiments, the methods and compositions disclosed herein may reduce amplification of undesirable nucleic acid species without significantly reducing the number of different molecular label sequences associated with the other nucleic acid target molecules. For example, the methods and compositions disclosed herein can reduce amplification of undesirable nucleic acid species while retaining at least at least 10%, at least 20%, at least 30%, at least 40%, at least 50%, at least 60%, at least 70%, at least 80%, at least 90%, at least 95%, or 100% of the different molecular label sequences associated with the other nucleic acid target molecules. In some embodiments, the methods and compositions disclosed herein can reduce amplification of undesirable nucleic acid species by at least at least 10%, at least 20%, at least 30%, at least 40%, at least 50%, at least 60%, at least 70%, at least 80%, at least 90%, at least 95%, or at least 99% while retaining at least at least 10%, at least 20%, at least 30%, at least 40%, at least 50%, at least 60%, at least 70%, at least 80%, at least 90%, at least 95%, or 100% of the different molecular label sequences associated with the other nucleic acid target molecules. In some embodiments, reducing amplification of undesirable nucleic acid species does not significantly reduce the number of different molecular label sequences associated with the other nucleic acid target molecules. 
     Extension 
     One or more extension reactions can be performed using the oligonucleotide probes that are hybridized to the plurality of nucleic acid target molecules to generate a plurality of extension products. In some embodiments, the oligonucleotide probes function as primers for the extension reaction, such as reverse transcription. The extension reactions can be performed with or without the presence of blocking oligonucleotides. In embodiments where blocking oligonucleotides are present in the extension reactions, the blocking oligonucleotides can reduce the extension of one or more undesirable nucleic acid species to which the blocking oligonucleotides specifically bind. In some embodiments, the blocking oligonucleotides do not significantly reduce the extension of the nucleic acid target molecules. 
     The plurality of nucleic acid target molecules can, in some embodiments, randomly associate with the oligonucleotide probes. Association can, for example, comprise hybridization of an oligonucleotide probe&#39;s binding region to a complementary portion of the target nucleic acid molecule (e.g., oligo dT sequence of the stochastic barcode can interact with a poly-A tail of a target nucleic acid molecule). The assay conditions used for hybridization (e.g. buffer pH, ionic strength, temperature, etc.) can be chosen to promote formation of specific, stable hybrids. 
     The disclosure provides for methods of associating a molecular label with a target nucleic acid using reverse transcription. 
     Amplification 
     In some embodiments, the methods disclosed herein can comprise amplifying a sample wherein the sample comprises a plurality of nucleic acid target molecules and one or more undesirable nucleic acid species, or amplifying the plurality of extension products to generate a plurality of amplicons. In some embodiments, one or more nucleic acid amplification reactions can be performed to create multiple copies of the target nucleic acid molecules or the extension products. In some embodiments, primers can be added for the amplification reaction, such as PCR. The amplification reactions can be performed in or without the presence of blocking oligonucleotides. In embodiments, where blocking oligonucleotides are present in the amplification reactions, the blocking oligonucleotides can reduce the amplification of one or more undesirable nucleic acid species to which the blocking oligonucleotides specifically bind. In some embodiments, the blocking oligonucleotides do not significantly reduce the amplification of the nucleic acid target molecules. 
     Amplification can be performed, in some embodiments, in a multiplexed manner, wherein multiple target nucleic acid sequences are amplified simultaneously. The amplification reaction can be used, for example, to add sequencing adaptors to the nucleic acid molecules. The amplification reactions can comprise amplifying at least a portion of a sample label, if present. The amplification reactions can comprise amplifying at least a portion of the cellular and/or molecular label. The amplification reactions can comprise amplifying at least a portion of a sample tag, a cell label, a spatial label, a molecular label, a target nucleic acid, or a combination thereof. The amplification reactions can, for example, comprise amplifying at least 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 97%, or 100% of the plurality of target nucleic acids. The method may further comprise conducting one or more cDNA synthesis reactions to produce one or more cDNA copies of target-barcode molecules comprising a sample label, a cell label, a spatial label, and/or a molecular label. 
     In some embodiments, amplification can be performed using a polymerase chain reaction (PCR). As used herein, “PCR” refers to a reaction for the in vitro amplification of specific DNA sequences by the simultaneous primer extension of complementary strands of DNA. As used herein, PCR encompass derivative forms of the reaction, including but not limited to, RT-PCR, real-time PCR, nested PCR, quantitative PCR, multiplexed PCR, digital PCR, and assembly PCR. 
     Amplification of the labeled nucleic acids can comprise non-PCR based methods. Examples of non-PCR based methods include, but are not limited to, multiple displacement amplification (MDA), transcription-mediated amplification (TMA), whole transcriptome amplification (WTA), whole genome amplification (WGA), nucleic acid sequence-based amplification (NASBA), strand displacement amplification (SDA), real-time SDA, rolling circle amplification, or circle-to-circle amplification. Other non-PCR-based amplification methods include multiple cycles of DNA-dependent RNA polymerase-driven RNA transcription amplification or RNA-directed DNA synthesis and transcription to amplify DNA or RNA targets, a ligase chain reaction (LCR), and a Qβ replicase (Qβ) method, use of palindromic probes, strand displacement amplification, oligonucleotide-driven amplification using a restriction endonuclease, an amplification method in which a primer is hybridized to a nucleic acid sequence and the resulting duplex is cleaved prior to the extension reaction and amplification, strand displacement amplification using a nucleic acid polymerase lacking 5′ exonuclease activity, rolling circle amplification, and ramification extension amplification (RAM). In some instances, the amplification may not produce circularized transcripts. 
     In some instances, the methods disclosed herein further comprise conducting a polymerase chain reaction on the labeled nucleic acid (e.g., labeled-RNA, labeled-DNA, labeled-cDNA) to produce a labeled amplicon. The labeled amplicon can, for example, be a double-stranded molecule. The double-stranded molecule can comprise a double-stranded RNA molecule, a double-stranded DNA molecule, or a RNA molecule hybridized to a DNA molecule. One or both of the strands of the double-stranded molecule may comprise a sample label, a spatial label, a cell label, and/or a molecular label. The labeled amplicon can be a single-stranded molecule. The single-stranded molecule can comprise DNA, RNA, or a combination thereof. The nucleic acids of the disclosure comprise synthetic or altered nucleic acids. 
     Amplification can, for example, comprise use of one or more non-natural nucleotides. Non-natural nucleotides may comprise photolabile or triggerable nucleotides. Examples of non-natural nucleotides can include, but are not limited to, peptide nucleic acid (PNA), morpholino and locked nucleic acid (LNA), as well as glycol nucleic acid (GNA) and threose nucleic acid (TNA). Non-natural nucleotides may be added to one or more cycles of an amplification reaction. The addition of the non-natural nucleotides can be, for example, used to identify products as specific cycles or time points in the amplification reaction. 
     As described herein, conducting the one or more amplification reactions can comprise the use of one or more primers. A primer can, for example, comprise at least 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, or 15 or more nucleotides. In some embodiments, the primer comprises at least 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, or 15 or more nucleotides. For example, the primer can comprise 12 to 15 nucleotides. The one or more primers can, for example, anneal to at least a portion of the plurality of labeled target nucleic acid molecules and oligonucleotides. For example, the one or more primers can anneal to the 3′ end or 5′ end of the plurality of labeled target nucleic acid molecules and oligonucleotides. The one or more primers can, in some embodiments, anneal to an internal region of the plurality of labeled target nucleic acid molecules and oligonucleotides. The internal region of a oligonucleotide or target nucleic acid molecule can be, for example, at least about 50, 100, 150, 200, 220, 230, 240, 250, 260, 270, 280, 290, 300, 310, 320, 330, 340, 350, 360, 370, 380, 390, 400, 410, 420, 430, 440, 450, 460, 470, 480, 490, 500, 510, 520, 530, 540, 550, 560, 570, 580, 590, 600, 650, 700, 750, 800, 850, 900, or 1000 nucleotides from the 3′ ends and/or 5′ end of the oligonucleotide or the target nucleic acid molecule. The one or more primers may comprise a fixed panel of primers. The one or more primers may comprise at least one or more custom primers. The one or more primers may comprise at least one or more control primers. The one or more primers may comprise at least one or more gene-specific primers. 
     The one or more primers can comprise any universal primer of the disclosure. The universal primer may anneal to a universal primer binding site. The one or more custom primers can, in some embodiments, anneal to a first sample label, a second sample label, a spatial label, a cell label, a molecular label, a target, or any combination thereof. The one or more primers may comprise a universal primer and a custom primer. 
     Any amplification scheme can be used in the methods of the present disclosure. For example, in one scheme, the first round PCR can amplify molecules (e.g., attached to the bead) using a gene specific primer and a primer against the universal Illumina sequencing primer 1 sequence. The second round of PCR can amplify the first PCR products using a nested gene specific primer flanked by Illumina sequencing primer 2 sequence, and a primer against the universal Illumina sequencing primer 1 sequence. The third round of PCR adds P5 and P7 and sample index to turn PCR products into an Illumina sequencing library. Sequencing using 150 bp×2 sequencing can reveal the cell label and molecular index on read 1, the gene on read 2, and the sample index on index 1 read. 
     Amplification can be performed in one or more rounds. In some instances there are multiple rounds of amplification. Amplification can comprise two or more rounds of amplification. The first amplification can be an extension off X′ to generate the gene specific region. The second amplification can occur when a sample nucleic hybridizes to the newly generated strand. 
     In some embodiments, hybridization does not need to occur at the end of a nucleic acid molecule. In some embodiments, a target nucleic acid within an intact strand of a longer nucleic acid is hybridized and amplified. For example a target within a longer section of genomic DNA or mRNA. A target can be more than 50 nt, more than 100 nt, or more that 1000 nt from one end (e.g., 5′ end or 3′ end) of a polynucleotide. 
     Sequencing 
     In some embodiments, the extension products and/or the amplification products disclosed herein may be used for sequencing. Any suitable sequencing method known in the art can be used, preferably high-throughput approaches. For example, cyclic array sequencing using platforms such as Roche 454, Illumina Solexa, ABI-SOLiD, ION Torrent, Complete Genomics, Pacific Bioscience, Helicos, or the Polonator platform, may also be utilized. Sequencing may comprise MiSeq sequencing and/or HiSeq sequencing. The selective extension and/or amplification methods disclosed herein can, in some embodiments, increase the efficiency of sequencing by decreasing the number of sequencing reads for the undesirable nucleic acid species. 
     In some embodiments, after using the selective extension and/or amplification methods described herein, the sequencing reads for the undesirable nucleic acid species are less than 50%, less than 40%, less than 30%, less than 20%, less than 10%, less than 5%, or less, of the total sequencing reads. In some embodiments, the sequencing reads for the undesirable nucleic acid species are less than 40% of the total sequencing reads. In some embodiments, the sequencing reads for the undesirable nucleic acid species are less than 30% of the total sequencing reads. In some embodiments, the sequencing reads for the undesirable nucleic acid species are less than 20% of the total sequencing reads. In some embodiments, the sequencing reads for the undesirable nucleic acid species are less than 10% of the total sequencing reads. In some embodiments, after using the selective extension and/or amplification methods described herein, the sequencing reads for the undesirable nucleic acid species are reduced to less than 60%, less than 50%, less than 40%, less than 30%, less than 20%, less than 10%, less than 5% of the sequencing reads for the undesirable nucleic acid without using the selective extension and/or amplification methods described herein. In some embodiments, after using the selective extension and/or amplification methods described herein, the sequencing reads for the undesirable nucleic acid species are reduced to, or to about, 60%, 50%, 40%, 30%, 20%, 10%, 5%, 2%, 1%, 0.5%, or a range between any two of these values, of the sequencing reads for the undesirable nucleic acid without using the selective extension and/or amplification methods described herein. 
     In some embodiments, the methods and compositions disclosed herein can improve sequencing efficiency by decreasing the sequencing reads:molecular label ratio of an undesirable nucleic acid species and/or increasing the sequencing reads:molecular label ratio of a nucleic acid target molecule. For example, the ratio of sequencing reads to molecular label for an undesirable nucleic acid species can be less than 20, less than 15, less than 10, less than 9, less than 8, less than 7, less than 6, less than 5, less than 4, less than 3, less than 2, or less than 1. In some embodiments, the ratio of sequencing reads to molecular label for an undesirable nucleic acid species is 20, 15, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1, or a range between any two of these values. 
     Kits 
     Disclosed herein are kits for selective amplification and/or extension of a plurality of nucleic acid target molecules in a sample, wherein the sample comprises a plurality of target nucleic acid species and one or more undesirable nucleic acid species. In some embodiments, the kit comprises a plurality of oligonucleotide probes, wherein each of the plurality of oligonucleotide probes comprises a molecular label sequence and a binding region; and a plurality of blocking oligonucleotides that specifically binds to a plurality of undesirable nucleic acid species in the sample, wherein each blocking oligonucleotide probe is unable to function as a primer for a reverse transcriptase or a polymerase. 
     In some embodiments, the kit further comprises a plurality of blocking oligonucleotides. The plurality of blocking oligonucleotides can, for example, specifically bind to at least 1, at least 2, at least 5, at least 10, at least 100, at least 1,000 or more undesirable nucleic acid species in the sample. In some embodiments, the blocking oligonucleotide can specifically bind to within 10 nt, 20 nt, 30 nt, 40 nt, 50 nt, 100 nt, 200 nt, 300 nt, 400 nt, 500 nt, 1,000 nt of the 5′ end of the one or more undesirable nucleic acid species. In some embodiments, the blocking oligonucleotide can specifically bind to within 10 nt, 20 nt, 30 nt, 40 nt, 50 nt, 100 nt, 200 nt, 300 nt, 400 nt, 500 nt, 1,000 nt of the 3′ end of the one or more undesirable nucleic acid species. In some embodiments, the blocking oligonucleotide can specifically bind to within 10 nt, 20 nt, 30 nt, 40 nt, 50 nt, 100 nt, 200 nt, 300 nt, 400 nt, 500 nt, 1,000 nt surrounding the middle point of the one or more undesirable nucleic acid species. 
     It is contemplated that the blocking oligonucleotide may reduce the amplification and/or extension of the undesirable nucleic acid species by forming a hybridization complex with the undesirable nucleic acid species having a high melting temperature (T m ), by not being able to function as a primer for a reverse transcriptase or a polymerase, a combination thereof, etc. In some embodiments, the blocking oligonucleotide can have a T m  that is, is about, is at least, 50° C., 60° C., 65° C., 70° C., 75° C., 80° C., 85° C., 90° C., 95° C., or a range between any two of the above values. In some embodiments, the blocking oligonucleotide may reduce the amplification and/or extension of the undesirable nucleic acid species by competing with the amplification and/or extension primers for hybridization with the undesirable nucleic acid species. 
     The blocking oligonucleotide can, in some embodiments, comprise one or more non-natural nucleotides. Non-natural nucleotides can comprise photolabile or triggerable nucleotides. Examples of non-natural nucleotides can include, but are not limited to, peptide nucleic acid (PNA), morpholino and locked nucleic acid (LNA), as well as glycol nucleic acid (GNA) and threose nucleic acid (TNA). In some embodiments, the blocking oligonucleotide is a chimeric oligonucleotide, such as an LNA/PNA/DNA chimera, an LNA/DNA chimera, a PNA/DNA chimera, a GNA/DNA chimera, a TNA/DNA chimera, and a combination thereof. 
     It would be appreciated that the T m  of a blocking oligonucleotide can be modified by adjusting the length of the blocking oligonucleotide. For example, a blocking oligonucleotide can have a length that is, is about, is less than, is more than, 10 nt, 15 nt, 20 nt, 25 nt, 30 nt, 35 nt, 40 nt, 45 nt, 50 nt, 60 nt, 70 nt, 80 nt, 90 nt, 100 nt, 200 nt, or a range between any two of the above values. 
     In some embodiments, the T m  of a blocking oligonucleotide can be modified by adjusting the number of DNA residues in the blocking oligonucleotide that comprises an LNA/DNA chimera or a PNA/DNA chimera. For example, a blocking oligonucleotide that comprises an LNA/DNA chimera or a PNA/DNA chimera can have a percentage of DNA residues that is, is about, is less than, is more than, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 60%, 70%, 80%, 90%, or a range between any two of the above values. 
     In some embodiments, a blocking oligonucleotide can be designed to being incapable of functioning as a primer for an extension or amplication. For example, the blocking oligonucleotide may be incapable of functioning as a primer for a reverse transcriptase, a polymerase, or both. For example, a blocking oligonucleotide that comprises an LNA/DNA chimera or a PNA/DNA chimera can be designed to have a certain percentage of LNA or PNA residues, or to have LNA or PNA residues at certain location(s), such as the 3′ end, the 5′ end, the internal region, or a combination thereof of the blocking oligonucleotide. In some embodiments, a blocking oligonucleotide that comprises an LNA/DNA chimera or a PNA/DNA chimera can have a percentage of LNA or PNA residues that is, is about, is less than, is more than, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 60%, 70%, 80%, 90%, or a range between any two of the above values. 
     In some embodiments, the blocking oligonucleotides can comprise an affinity moiety. The affinity moiety can be a functional group selected from the group consisting of biotin, streptavidin, heparin, an aptamer, a click-chemistry moiety, digoxigenin, primary amine(s), carboxyl(s), hydroxyl(s), aldehyde(s), ketone(s), and any combination thereof. In some embodiments, the blocking oligonucleotides can be immobilized to a solid support having a binding partner for the affinity moiety through the affinity moiety. 
     In some embodiments, each of the oligonucleotide probes can comprise a molecular label, a cell label, a sample label, or any combination thereof. In some embodiments, each of the oligonucleotides can comprise a linker. In some embodiments, each of the oligonucleotide probes can comprise a binding site for an oligonucleotide probe, such as a poly A tail. For example, the poly A tail can be, e.g., oligodA 18  (unanchored to a solid support) or oligoA 18 V (anchored to a solid support). The oligonucleotide probes can comprise DNA residues, RNA residues, or both. 
     In some embodiments, the kits can further comprise an enzyme. In some embodiments, the enzyme can be a reverse transcriptase, a polymerase, a ligase, a nuclease, or, any combination thereof. 
     Stochastic Barcodes 
     The oligonucleotide probes disclosed herein can comprise, or consists of, stochastic barcodes. As disclosed herein, a stochastic barcode can be a polynucleotide sequence that may be used to stochastically label (e.g., barcode, tag) a target. A stochastic barcode can comprise one or more labels. Exemplary labels include, but are not limited to, universal labels, cell labels, molecular labels, sample labels, plate labels, spatial labels, pre-spatial labels, and any combination thereof. A stochastic barcode can comprise a 5′amine that may link the stochastic barcode to a solid support. The stochastic barcode can comprise one or more of a universal label, a dimension label, a spatial label, a cell label, and a molecular label. The universal label can be 5′-most label. The molecular label can be the 3′-most label. The spatial label, dimension label, and the cell label can be in any order. In some instances, the universal label, the spatial label, the dimension label, the cell label, and the molecular label are in any order. The stochastic barcode can comprise a target-binding region. The target-binding region can interact with a target (e.g., target nucleic acid, RNA, mRNA, DNA) in a sample. For example, a target-binding region can comprise an oligo dT sequence which can interact with poly-A tails of mRNAs. In some instances, the labels of the stochastic barcode (e.g., universal label, dimension label, spatial label, cell label, and molecular label) may be separated by 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, a range between any two of these values, or more nucleotides. 
     A stochastic barcode can comprise one or more universal labels. The one or more universal labels can be the same for all stochastic barcodes in the set of stochastic barcodes (e.g., attached to a given solid support). In some embodiments, the one or more universal labels can be the same for all stochastic barcodes attached to a plurality of beads. In some embodiments, a universal label comprises a nucleic acid sequence that is capable of hybridizing to a sequencing primer. Sequencing primers can be used for sequencing stochastic barcodes comprising a universal label. Sequencing primers (e.g., universal sequencing primers) can comprise sequencing primers associated with high-throughput sequencing platforms. In some embodiments, a universal label may comprise a nucleic acid sequence that is capable of hybridizing to a PCR primer. In some embodiments, the universal label comprises a nucleic acid sequence that is capable of hybridizing to a sequencing primer and a PCR primer. The nucleic acid sequence of the universal label that is capable of hybridizing to a sequencing or PCR primer may be referred to as a primer binding site. A universal label can comprise a sequence that may be used to initiate transcription of the stochastic barcode. A universal label can comprise a sequence that may be used for extension of the stochastic barcode or a region within the stochastic barcode. A universal label can be, or be at least about, 1, 2, 3, 4, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50 or more nucleotides in length. A universal label can comprise at least about 10 nucleotides. A universal label can be at most about 1, 2, 3, 4, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50 or more nucleotides in length. In some embodiments, a cleavable linker or modified nucleotide is part of the universal label sequence to enable the stochastic barcode to be cleaved off from the support. As used herein, a universal label can be used interchangeably with “universal PCR primer.” 
     A stochastic barcode can comprise a dimension label. A dimension label can comprise a nucleic acid sequence that provides information about a dimension in which the stochastic labeling occurred. For example, a dimension label can provide information about the time at which a target was stochastically barcoded. A dimension label can be associated with a time of stochastic barcoding in a sample. A dimension label can activated at the time of stochastic labeling. Different dimension labels can be activated at different times. The dimension label provides information about the order in which targets, groups of targets, and/or samples were stochastically barcoded. For example, a population of cells can be stochastically barcoded at the G0 phase of the cell cycle. The cells can be pulsed again with stochastic barcodes at the G1 phase of the cell cycle. The cells can be pulsed again with stochastic barcodes at the S phase of the cell cycle, and so on. Stochastic barcodes at each pulse (e.g., each phase of the cell cycle), can comprise different dimension labels. In this way, the dimension label provides information about which targets were labelled at which phase of the cell cycle. Dimension labels can interrogate many different biological times. Exemplary biological times can include, but are not limited to, the cell cycle, transcription (e.g., transcription initiation), and transcript degradation. In another example, a sample (e.g., a cell, a population of cells) can be stochastically labeled before and/or after treatment with a drug and/or therapy. The changes in the number of copies of distinct targets can be indicative of the sample&#39;s response to the drug and/or therapy. 
     In some embodiments, a dimension label is activatable. An activatable dimension label can be activated, for example, at a specific timepoint. The activatable dimension label can be constitutively activated (e.g., not turned off). The activatable dimension label can be reversibly activated (e.g., the activatable dimension label can be turned on and turned off). The dimension label can be reversibly activatable at least 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10 or more times. The dimension label can be reversibly activatable 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10 or more times. For example, the dimension label can be activated with fluorescence, light, a chemical event (e.g., cleavage, ligation of another molecule, addition of modifications (e.g., pegylated, sumoylated, acetylated, methylated, deacetylated, demethylated), a photochemical event (e.g., photocaging), and introduction of a non-natural nucleotide. 
     The dimension label can be identical for all stochastic barcodes attached to a given solid support (e.g., bead), but different for different solid supports (e.g., beads). In some embodiments, at least 60%, 70%, 80%, 85%, 90%, 95%, 97%, 99% or 100% of stochastic barcodes on the same solid support comprise the same dimension label. In some embodiments, at least 60% of stochastic barcodes on the same solid support comprise the same dimension label. In some embodiments, at least 95% of stochastic barcodes on the same solid support comprise the same dimension label. 
     There can be as many as 10 6  or more unique dimension label sequences represented in a plurality of solid supports (e.g., beads). A dimension label can, for example, be or be at least about 1, 2, 3, 4, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50 or more nucleotides in length. A dimension label can be at most about 300, 200, 100, 90, 80, 70, 60, 50, 40, 30, 20, 15, 12, 10, 9, 8, 7, 6, 5, 4 or fewer or more nucleotides in length. A dimension label can, for example, is about 5 to about 200 nucleotides, or about 10 to about 150 nucleotides in length. In some embodiments, a dimension label is from about 20 to about 125 nucleotides in length. 
     A stochastic barcode can comprise a spatial label. A spatial label can comprise a nucleic acid sequence that provides information about the spatial orientation of a target molecule which is associated with the stochastic barcode. A spatial label can be associated with a coordinate in a sample. The coordinate can be a fixed coordinate. For example a coordinate can be fixed in reference to a substrate. A spatial label can be in reference to a two or three-dimensional grid. A coordinate can be fixed in reference to a landmark. The landmark can be identifiable in space. A landmark can be a structure which can be imaged. A landmark can be a biological structure, for example an anatomical landmark. A landmark can be a cellular landmark, for instance an organelle. A landmark can be a non-natural landmark such as a structure with an identifiable identifier such as a color code, bar code, magnetic property, fluorescents, radioactivity, or a unique size or shape. A spatial label can be associated with a physical partition (e.g. a well, a container, or a droplet). In some instances, multiple spatial labels are used together to encode one or more positions in space. 
     The spatial label can be identical for all stochastic barcodes attached to a given solid support (e.g., bead), but different for different solid supports (e.g., beads). In some embodiments, at least 60%, 70%, 80%, 85%, 90%, 95%, 97%, 99% or 100% of stochastic barcodes on the same solid support comprise the same spatial label. In some embodiments, at least 60% of stochastic barcodes on the same solid support comprise the same spatial label. In some embodiments, at least 95% of stochastic barcodes on the same solid support comprise the same spatial label. 
     There can be as many as 10 6  or more unique spatial label sequences represented in a plurality of solid supports (e.g., beads). A spatial label can be, or be at least about, 1, 2, 3, 4, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50 or more nucleotides in length. In some embodiments, a spatial label is most about 300, 200, 100, 90, 80, 70, 60, 50, 40, 30, 20, 15, 12, 10, 9, 8, 7, 6, 5, 4 nucleotides in length. A spatial label can be, for example, from about 5 to about 200 nucleotides in length. A spatial label can be, for example, from about 10 to about 150 nucleotides in length. A spatial label can be from about 20 to about 125 nucleotides in length. 
     Stochastic barcodes can comprise a cell label. A cell label can comprise a nucleic acid sequence that provides information for determining which target nucleic acid originated from which cell. In some embodiments, the cell label is identical for all stochastic barcodes attached to a given solid support (e.g., bead), but different for different solid supports (e.g., beads). In some embodiments, at least 60%, 70%, 80%, 85%, 90%, 95%, 97%, 99% or 100% of stochastic barcodes on the same solid support comprise the same cell label. In some embodiments, at least 60% of stochastic barcodes on the same solid support comprise the same cell label. In some embodiment, at least 95% of stochastic barcodes on the same solid support comprise the same cell label. 
     There can be as many as 10 6  or more unique cell label sequences represented in a plurality of solid supports (e.g., beads). A cell label may be at least about 1, 2, 3, 4, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50 or more nucleotides in length. A cell label can be, or be at most about, 300, 200, 100, 90, 80, 70, 60, 50, 40, 30, 20, 15, 12, 10, 9, 8, 7, 6, 5, 4 or fewer or more nucleotides in length. A cell label can be, for example, from about 5 to about 200 nucleotides in length. A cell label can be, for example, from about 10 to about 150 nucleotides in length. A cell label can be, for exmaple, from about 20 to about 125 nucleotides in length. 
     Stochastic barcodes can comprise a molecular label. A molecular label can comprise a nucleic acid sequence that provides identifying information for the specific type of target nucleic acid species hybridized to the stochastic barcode. A molecular label can comprise a nucleic acid sequence that provides a counter for the specific occurrence of the target nucleic acid species hybridized to the stochastic barcode (e.g., target-binding region). In some embodiments, a diverse set of molecular labels are attached to a given solid support (e.g., bead). In some embodiments, there can be as many as 10 6  or more unique molecular label sequences attached to a given solid support (e.g., bead). In some embodiments, there can be as many as 10 5  or more unique molecular label sequences attached to a given solid support (e.g., bead). In some embodiments, there can be as many as 10 4  or more unique molecular label sequences attached to a given solid support (e.g., bead). In some embodiments, there can be as many as 10 3  or more unique molecular label sequences attached to a given solid support (e.g., bead). In some embodiments, there can be as many as 10 2  or more unique molecular label sequences attached to a given solid support (e.g., bead). A molecular label can be at least about 1, 2, 3, 4, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50 or more nucleotides in length. A molecular label can be at most about 300, 200, 100, 90, 80, 70, 60, 50, 40, 30, 20, 15, 12, 10, 9, 8, 7, 6, 5, 4 or fewer nucleotides in length. 
     Stochastic barcodes can comprise a target binding region. In some embodiments, the target binding regions comprise a nucleic acid sequence that hybridizes specifically to a target (e.g., target nucleic acid, target molecule, e.g., a cellular nucleic acid to be analyzed), for example to a specific gene sequence. In some embodiments, a target binding region comprise a nucleic acid sequence that may attach (e.g., hybridize) to a specific location of a specific target nucleic acid. In some embodiments, the target binding region comprise a nucleic acid sequence that is capable of specific hybridization to a restriction site overhang (e.g. an EcoRI sticky-end overhang). The stochastic barcode may then ligate to any nucleic acid molecule comprising a sequence complementary to the restriction site overhang. 
     A stochastic barcode can comprise a target-binding region. A target-binding region can hybridize with a target of interest. For example, a target-binding region can comprise an oligo dT which can hybridize with mRNAs comprising poly-adenylated ends. A target-binding region can be gene-specific. For example, a target-binding region can be configured to hybridize to a specific region of a target. A target-binding region can be, or be at least, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26 27, 28, 29, or 30 or more nucleotides in length. A target-binding region can be at most 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26 27, 28, 29, or 30 or more nucleotides in length. A target-binding region can be from 5-30 nucleotides in length. When a stochastic barcode comprises a gene-specific target-binding region, the stochastic barcode can be referred to as a gene-specific stochastic barcode. 
     A target binding region can comprise a non-specific target nucleic acid sequence. A non-specific target nucleic acid sequence can refer to a sequence that may bind to multiple target nucleic acids, independent of the specific sequence of the target nucleic acid. For example, target binding region can comprise a random multimer sequence, or an oligo-dT sequence that hybridizes to the poly-A tail on mRNA molecules. A random multimer sequence can be, for example, a random dimer, trimer, quatramer, pentamer, hexamer, septamer, octamer, nonamer, decamer, or higher multimer sequence of any length. In some embodiments, the target binding region is the same for all stochastic barcodes attached to a given bead. In some embodiments, the target binding regions for the plurality of stochastic barcodes attached to a given bead comprise two or more different target binding sequences. A target binding region can be, or be at least about, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50 or more nucleotides in length. In some embodiments, a target binding region is at most about 5, 10, 15, 20, 25, 30, 35, 40, 45, 50 or more nucleotides in length. 
     A stochastic barcode can comprise an orientation property which can be used to orient (e.g., align) the stochastic barcodes. A stochastic barcode can comprise a moiety for isoelectric focusing. Different stochastic barcodes can comprise different isoelectric focusing points. When these stochastic barcodes are introduced to a sample, the sample can undergo isoelectric focusing in order to orient the stochastic barcodes into a known way. In this way, the orientation property can be used to develop a known map of stochastic barcodes in a sample. Exemplary orientation properties include, but are not limited to, electrophoretic mobility (e.g., based on size of the stochastic barcode), isoelectric point, spin, conductivity, and/or self-assembly. For example, stochastic barcodes can comprise an orientation property of self-assembly, can self-assemble into a specific orientation (e.g., nucleic acid nanostructure) upon activation. 
     The cell label and/or any label of the disclosure can further comprise a unique set of nucleic acid sub-sequences of defined length, e.g. 7 nucleotides each (equivalent to the number of bits used in some Hamming error correction codes), which are designed to provide error correction capability. Hamming codes, like other error-correcting codes, are based on the principle of redundancy and can be constructed by adding redundant parity bits to data that is to be transmitted over a noisy medium. Such error-correcting codes can encode sample identifiers with redundant parity bits, and “transmit” these sample identifiers as codewords. A Hamming code can refer an arithmetic process that identifies unique binary codes based upon inherent redundancy that are capable of correcting single bit errors. For example, a Hamming code can be matched with a nucleic acid barcode in order to screen for single nucleotide errors occurring during nucleic acid amplification. The identification of a single nucleotide error by using a Hamming code, thereby can allow for the correction of the nucleic acid barcode. 
     Hamming codes can be represented by a subset of the possible codewords that are chosen from the center of multidimensional spheres (i.e., for example, hyperspheres) in a binary subspace. Single bit errors may fall within hyperspheres associated with a specific codeword and can thus be corrected. On the other hand, double bit errors that do not associate with a specific codeword can be detected, but not corrected. Consider a first hypersphere centered at coordinates (0, 0, 0) (i.e., for example, using an x-y-z coordinate system), wherein any single-bit error can be corrected by falling within a radius of 1 from the center coordinates; i.e., for example, single bit errors having the coordinates of (0, 0, 0); (0, 1, 0); (0, 0, 1); (1, 0, 0), or (1, 1, 0). Likewise, a second hypersphere may be constructed wherein single-bit errors can be corrected by falling within a radius of  1  of its center coordinates (1, 1, 1) (i.e., for example, (1,1,1); (1, 0, 1); (0,1, 0); or (0, 1, 1). 
     In some embodiments, the length of the nucleic acid sub-sequences used for creating error correction codes can vary, for example, they can be at least 3 nucleotides, at least 7 nucleotides, at least 15 nucleotides, or at least 31 nucleotides in length. In some embodiments, nucleic acid sub-sequences of other lengths can be used for creating error correction codes. 
     When a stochastic barcode comprises more than one of a type of label (e.g., more than one cell label or more than one molecular label), the labels may be interspersed with a linker label sequence. A linker label sequence can be at least about 5, 10, 15, 20, 25, 30, 35, 40, 45, 50 or more nucleotides in length. A linker label sequence can be at most about 5, 10, 15, 20, 25, 30, 35, 40, 45, 50 or more nucleotides in length. In some instances, a linker label sequence is 12 nucleotides in length. A linker label sequence can be used to facilitate the synthesis of the stochastic barcode. The linker label can comprise an error-correcting (e.g., Hamming) code. 
     Solid Supports 
     The oligonucleotide probes (e.g., stochastic barcodes) and/or the blocking oligonucleotides disclosed herein can, in some embodiments, be attached to a solid support (e.g., bead, substrate, microwell(s), microwell arrays). As used herein, the terms “tethered”, “attached”, and “immobilized” are used interchangeably, and refer to covalent or non-covalent means for attaching a compound (e.g., an oligonucleotide) to a solid support. Any of a variety of different solid supports may be used as solid supports for attaching pre-synthesized combinatorial barcode reagents or for in situ solid-phase synthesis of combinatorial barcode reagents. 
     The solid support can be or comprise, for example, a particle or a plurality of particles. The particles can be, for example, nanoparticles, microparticles, or the likes. In some embodiments, a solid support is, or comprises, a bead or a plurality of beads. The particle (e.g., the bead) can encompass any type of solid, porous, or hollow sphere, ball, bearing, cylinder, or other similar configuration composed of plastic, ceramic, metal, or polymeric material onto which a nucleic acid may be immobilized (e.g., covalently or non-covalently). The particle (e.g., the bead) can comprise a discrete particle that may be spherical (e.g., microspheres) or have a non-spherical or irregular shape, such as cubic, cuboid, pyramidal, cylindrical, conical, oblong, or disc-shaped, and the like. The particle (e.g., the bead) can be spherical, substantial spherical, or non-spherical in shape. 
     The particle (e.g., bead) can comprise a variety of materials including, but not limited to, paramagnetic materials (e.g. magnesium, molybdenum, lithium, and tantalum), superparamagnetic materials (e.g. ferrite (Fe 3 O 4 ; magnetite) nanoparticles), ferromagnetic materials (e.g. iron, nickel, cobalt, some alloys thereof, and some rare earth metal compounds), ceramic, plastic, glass, polystyrene, silica, methylstyrene, acrylic polymers, titanium, latex, sepharose, agarose, hydrogel, polymer, cellulose, nylon, and any combination thereof. 
     The diameter of the particle (e.g., the bead) can be, or be at least about, 5 μm, 10 μm, 20 μm, 25 μm, 30 μm, 35 μm, 40 μm, 45 μm, 50 μm, or in a range of any two of these values. The diameter of the particle (e.g., the bead) can be, for examples, at most about 5 μm, 10 μm, 20 μm, 25 μm, 30 μm, 35 μm, 40 μm, 45 μm or 50 μm. The diameter of the particle (e.g., the bead) can be related to the diameter of the wells of the substrate. For example, the diameter of the particle (e.g., bead) can be, or be at least, 10, 20, 30, 40, 50, 60, 70, 80, 90 or 100% longer or shorter than the diameter of the well. In some embodiments, the diameter of the particle (e.g., bead) can be at most 10, 20, 30, 40, 50, 60, 70, 80, 90 or 100% longer or shorter than the diameter of the well. The diameter of the particle (e.g., bead) can be related to the diameter of a cell (e.g., a single cell entrapped by a well of the substrate). The diameter of the particle (e.g., bead) can be, or be at least, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 150, 200, 250, or 300% or more longer or shorter than the diameter of the cell. In some embodiments, the diameter of the particle (e.g., bead) can be at most 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 150, 200, 250, or 300% or more longer or shorter than the diameter of the cell. 
     A particle (e.g., bead) can be attached to and/or embedded in a substrate. For example, the particle (e.g., bead) can be attached to and/or embedded in a gel, hydrogel, polymer and/or matrix. The spatial position of the particle (e.g., bead) within a substrate (e.g., gel, matrix, scaffold, or polymer) can be identified, in some embodiments, using the spatial label present on the stochastic barcode on the bead which can serve as a location address. 
     Examples of the particles (e.g., beads) can include, but are not limited to, streptavidin beads, agarose beads, magnetic beads, Dynabeads®, MACS® microbeads, antibody conjugated beads (e.g., anti-immunoglobulin microbead), protein A conjugated beads, protein G conjugated beads, protein A/G conjugated beads, protein L conjugated beads, oligodT conjugated beads, silica beads, silica-like beads, anti-biotin microbead, anti-fluorochrome microbead, and BcMag™ Carboxy-Terminated Magnetic Beads. 
     A particle (e.g., bead) can be associated with (e.g. impregnated with) quantum dots or fluorescent dyes to make it fluorescent in one fluorescence optical channel or multiple optical channels. A bead can be associated with iron oxide or chromium oxide to make it paramagnetic or ferromagnetic. The particles (e.g., beads) can be identifiable. A particle (e.g., bead) can be imaged using a camera. A particle (e.g., bead) can have a detectable code associated with the bead. For example, a bead can comprise an RFID tag. A bead can comprise any detectable tag (e.g., UPC code, electronic barcode, etched identifier). A particle (e.g., bead) can change size, for example due to swelling in an organic or inorganic solution. A bead can be hydrophobic or hydrophilic. A particle (e.g., bead) can be biocompatible. 
     A solid support (e.g., bead) can be visualized. The solid support can comprise a visualizing tag (e.g., fluorescent dye). A solid support (e.g., bead) can be etched with an identifier (e.g., a number). The identifier can be visualized through imaging the solid supports (e.g., beads). 
     A solid support (e.g., bead) can comprise an insoluble, semi-soluble, or insoluble material. A solid support can be referred to as “functionalized” when it includes a linker, a scaffold, a building block, or other reactive moiety attached thereto, whereas a solid support may be “nonfunctionalized” when it lack such a reactive moiety attached thereto. The solid support can be employed free in solution, such as in a microtiter well format; in a flow-through format, such as in a column; or in a dipstick. 
     The solid support can comprise a membrane, paper, plastic, coated surface, flat surface, glass, slide, chip, or any combination thereof. A solid support can take the form of resins, gels, microspheres, or other geometric configurations. A solid support can comprise silica chips, microparticles, nanoparticles, plates, arrays, capillaries, flat supports such as glass fiber filters, glass surfaces, metal surfaces (steel, gold silver, aluminum, silicon and copper), glass supports, plastic supports, silicon supports, chips, filters, membranes, microwell plates, slides, plastic materials including multiwell plates or membranes (e.g., formed of polyethylene, polypropylene, polyamide, polyvinylidenedifluoride), and/or wafers, combs, pins or needles (e.g., arrays of pins suitable for combinatorial synthesis or analysis) or beads in an array of pits or nanoliter wells of flat surfaces such as wafers (e.g., silicon wafers), wafers with pits with or without filter bottoms. 
     The solid support can comprise a polymer matrix (e.g., gel, hydrogel). The polymer matrix may be able to permeate intracellular space (e.g., around organelles). The polymer matrix may able to be pumped throughout the circulatory system. 
     A solid support can comprise, or be, a biological molecule. For example, a solid support can be a nucleic acid, a protein, an antibody, a histone, a cellular compartment, a lipid, a carbohydrate, and the like. Solid supports that are biological molecules can be amplified, translated, transcribed, degraded, and/or modified (e.g., pegylated, sumoylated, acetylated, methylated). A solid support that is a biological molecule can provide spatial and time information in addition to the spatial label that is attached to the biological molecule. For example, a biological molecule can comprise a first confirmation when unmodified, but can change to a second confirmation when modified. The different conformations can expose stochastic barcodes of the disclosure to targets. For example, a biological molecule can comprise stochastic barcodes that are unaccessible due to folding of the biological molecule. Upon modification of the biological molecule (e.g., acetylation), the biological molecule can change conformation to expose the stochastic labels. The timing of the modification can provide another time dimension to the method of stochastic barcoding of the disclosure. 
     In some embodiments, the biological molecule comprising combinatorial barcode reagents of the disclosure can be located in the cytoplasm of a cell. Upon activation, the biological molecule can move to the nucleus, whereupon stochastic barcoding can take place. In this way, modification of the biological molecule can encode additional space-time information for the targets identified by the stochastic barcodes. 
     A dimension label can provide information about space-time of a biological event (e.g., cell division). For example, a dimension label can be added to a first cell, where the first cell can divide generating a second daughter cell, the second daughter cell can comprise all, some or none of the dimension labels. The dimension labels can be activated in the original cell and the daughter cell. In this way, the dimension label can provide information about time of combinatorial barcoding in distinct spaces. 
     Substrates 
     As used herein, a substrate can refer to a type of solid support. A substrate can refer to a solid support that can comprise combinatorial barcode reagents of the disclosure. A substrate can comprise a plurality of microwells. A microwell can comprise a small reaction chamber of defined volume. A microwell can entrap one or more cells. A microwell can entrap only one cell. A microwell can entrap one or more solid supports. A microwell can entrap only one solid support. In some instances, a microwell entraps a single cell and a single solid support (e.g., bead). A microwell can comprise combinatorial barcode reagents of the disclosure. 
     The microwells of the array can be fabricated in a variety of shapes and sizes. Well geometries can include, but are not limited to, cylindrical, conical, hemispherical, rectangular, or polyhedral (e.g., three dimensional geometries comprised of several planar faces, for example, hexagonal columns, octagonal columns, inverted triangular pyramids, inverted square pyramids, inverted pentagonal pyramids, inverted hexagonal pyramids, or inverted truncated pyramids). The microwells can comprise a shape that combines two or more of these geometries. For example, a microwell can be partly cylindrical, with the remainder having the shape of an inverted cone. A microwell can include two side-by-side cylinders, one of larger diameter (e.g. that corresponds roughly to the diameter of the beads) than the other (e.g. that corresponds roughly to the diameter of the cells), that are connected by a vertical channel (that is, parallel to the cylinder axes) that extends the full length (depth) of the cylinders. The opening of the microwell can be at the upper surface of the substrate. The opening of the microwell can be at the lower surface of the substrate. The closed end (or bottom) of the microwell can be flat. The closed end (or bottom) of the microwell can have a curved surface (e.g., convex or concave). The shape and/or size of the microwell can be determined based on the types of cells or solid supports to be trapped within the microwells. 
     The portion of the substrate between the wells can have a topology. For example, the portion of the substrate between the wells can be rounded. The portion of the substrate between the wells can be pointed. The spacing portion of the substrate between the wells can be flat. The portion of the substrate between the wells may not be flat. In some instances, the portion of the substrate between wells is rounded. In other words, the portion of the substrate that does not comprise a well can have a curved surface. The curved surface can be fabricated such that the highest point (e.g., apex) of the curved surface may be at the furthest point between the edges of two or more wells (e.g., equidistant from the wells). The curved surface can be fabricated such that the start of the curved surface is at the edge of a first microwell and creates a parabola that ends at the end of a second microwell. This parabola can be extended in 2 dimensions to capture microwells nearby on the hexagonal grid of wells. The curved surface can be fabricated such that the surface between the wells is higher and/or curved than the plane of the opening of the well. The height of the curved surface can be, or be at least, 0.1, 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, or 7 or more micrometers. In some embodiments, the height of the curved surface can be at most 0.1, 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, or 7 or more micrometers. 
     Microwell dimensions can be characterized in terms of the diameter and depth of the well. As used herein, the diameter of the microwell refers to the largest circle that can be inscribed within the planar cross-section of the microwell geometry. The diameter of the microwells can range from about 1-fold to about 10-fold the diameter of the cells or solid supports to be trapped within the microwells. The microwell diameter can be, or be at least, 1-fold, at least 1.5-fold, at least 2-fold, at least 3-fold, at least 4-fold, at least 5-fold, or at least 10-fold the diameter of the cells or solid supports to be trapped within the microwells. In some embodiments, the microwell diameter can be at most 10-fold, at most 5-fold, at most 4-fold, at most 3-fold, at most 2-fold, at most 1.5-fold, or at most 1-fold the diameter of the cells or solid supports to be trapped within the microwells. The microwell diameter can be about 2.5-fold the diameter of the cells or solid supports to be trapped within the microwells. 
     The diameter of the microwells can be specified in terms of absolute dimensions. The diameter of the microwells can range from about 5 to about 60 micrometers. The microwell diameter can be, or be at least, 5 micrometers, at least 10 micrometers, at least 15 micrometers, at least 20 micrometers, at least 25 micrometers, at least 30 micrometers, at least 35 micrometers, at least 40 micrometers, at least 45 micrometers, at least 50 micrometers, or at least 60 micrometers. The microwell diameter can be at most 60 micrometers, at most 50 micrometers, at most 45 micrometers, at most 40 micrometers, at most 35 micrometers, at most 30 micrometers, at most 25 micrometers, at most 20 micrometers, at most 15 micrometers, at most 10 micrometers, or at most 5 micrometers. The microwell diameter can be about 30 micrometers. 
     The microwell depth may be chosen to provide efficient trapping of cells and solid supports. The microwell depth may be chosen to provide efficient exchange of assay buffers and other reagents contained within the wells. The ratio of diameter to height (i.e. aspect ratio) may be chosen such that once a cell and solid support settle inside a microwell, they will not be displaced by fluid motion above the microwell. The dimensions of the microwell may be chosen such that the microwell has sufficient space to accommodate a solid support and a cell of various sizes without being dislodged by fluid motion above the microwell. The depth of the microwells can range from about 1-fold to about 10-fold the diameter of the cells or solid supports to be trapped within the microwells. The microwell depth can be, or be at least, 1-fold, at least 1.5-fold, at least 2-fold, at least 3-fold, at least 4-fold, at least 5-fold, or at least 10-fold the diameter of the cells or solid supports to be trapped within the microwells. The microwell depth can be at most 10-fold, at most 5-fold, at most 4-fold, at most 3-fold, at most 2-fold, at most 1.5-fold, or at most 1-fold the diameter of the cells or solid supports to be trapped within the microwells. The microwell depth can be about 2.5-fold the diameter of the cells or solid supports to be trapped within the microwells. 
     The depth of the microwells can be specified in terms of absolute dimensions. The depth of the microwells may range from about 10 to about 60 micrometers. The microwell depth can be, or be at least, 10 micrometers, at least 20 micrometers, at least 25 micrometers, at least 30 micrometers, at least 35 micrometers, at least 40 micrometers, at least 50 micrometers, or at least 60 micrometers. The microwell depth can be at most 60 micrometers, at most 50 micrometers, at most 40 micrometers, at most 35 micrometers, at most 30 micrometers, at most 25 micrometers, at most 20 micrometers, or at most 10 micrometers. The microwell depth can be about 30 micrometers. 
     The volume of the microwells used in the methods, devices, and systems of the present disclosure can range from about 200 micrometers 3  to about 120,000 micrometers 3 . The microwell volume can be at least 200 micrometers 3 , at least 500 micrometers 3 , at least 1,000 micrometers 3 , at least 10,000 micrometers 3 , at least 25,000 micrometers 3 , at least 50,000 micrometers 3 , at least 100,000 micrometers 3 , or at least 120,000 micrometers 3 . The microwell volume can be at most 120,000 micrometers 3 , at most 100,000 micrometers 3 , at most 50,000 micrometers 3 , at most 25,000 micrometers 3 , at most 10,000 micrometers 3 , at most 1,000 micrometers 3 , at most 500 micrometers 3 , or at most 200 micrometers 3 . The microwell volume can be about 25,000 micrometers 3 . The microwell volume may fall within any range bounded by any of these values (e.g. from about 18,000 micrometers 3  to about 30,000 micrometers 3 ). 
     The volume of the microwell can be, or be at least, 5, 10, 15, 20, 25, 30, 35 40, 45 or 50 or more nanoliters 3 . The volume of the microwell can be at most 5, 10, 15, 20, 25, 30, 35 40, 45 or 50 or more nanoliters 3 . The volume of liquid that can fit in the microwell can be at least 5, 10, 15, 20, 25, 30, 35 40, 45 or 50 or more nanoliters 3 . The volume of liquid that can fit in the microwell can be at most 5, 10, 15, 20, 25, 30, 35 40, 45 or 50 or more nanoliters 3 . The volume of the microwell can be, or be at least, 5, 10, 15, 20, 25, 30, 35 40, 45 or 50 or more picoliters 3 . The volume of the microwell can be at most 5, 10, 15, 20, 25, 30, 35 40, 45 or 50 or more picoliters 3 . The volume of liquid that can fit in the microwell can be at least 5, 10, 15, 20, 25, 30, 35 40, 45 or 50 or more picoliters 3 . The volume of liquid that can fit in the microwell can be at most 5, 10, 15, 20, 25, 30, 35 40, 45 or 50 or more picoliters 3 . 
     The volumes of the microwells used in the methods, devices, and systems of the present disclosure may be further characterized in terms of the variation in volume from one microwell to another. The coefficient of variation (expressed as a percentage) for microwell volume may range from about 1% to about 10%. The coefficient of variation for microwell volume may be at least 1%, at least 2%, at least 3%, at least 4%, at least 5%, at least 6%, at least 7%, at least 8%, at least 9%, or at least 10%. The coefficient of variation for microwell volume may be at most 10%, at most 9%, at most 8%, at most 7%, at most 6%, at most 5%, at most 4%, at most 3%, at most 2%, or at most 1%. The coefficient of variation for microwell volume may have any value within a range encompassed by these values, for example between about 1.5% and about 6.5%. In some embodiments, the coefficient of variation of microwell volume may be about 2.5%. 
     The ratio of the volume of the microwells to the surface area of the beads (or to the surface area of a solid support to which stochastic barcode oligonucleotides may be attached) used in the methods, devices, and systems of the present disclosure can range from about 2.5 to about 1,520 micrometers. The ratio can be at least 2.5, at least 5, at least 10, at least 100, at least 500, at least 750, at least 1,000, or at least 1,520. The ratio can be at most 1,520, at most 1,000, at most 750, at most 500, at most 100, at most 10, at most 5, or at most 2.5. The ratio can be about 67.5. The ratio of microwell volume to the surface area of the bead (or solid support used for immobilization) may fall within any range bounded by any of these values (e.g. from about 30 to about 120). 
     The wells of the microwell array can be arranged in a one dimensional, two dimensional, or three-dimensional array. In some embodiments, a three dimensional array can be achieved, for example, by stacking a series of two or more two dimensional arrays (that is, by stacking two or more substrates comprising microwell arrays). 
     The pattern and spacing between microwells can be chosen to optimize the efficiency of trapping a single cell and single solid support (e.g., bead) in each well, as well as to maximize the number of wells per unit area of the array. The microwells may be distributed according to a variety of random or non-random patterns. For example, they may be distributed entirely randomly across the surface of the array substrate, or they may be arranged in a square grid, rectangular grid, hexagonal grid, or the like. In some instances, the microwells are arranged hexagonally. The center-to-center distance (or spacing) between wells may vary from about 5 micrometers to about 75 micrometers. In some instances, the spacing between microwells is about 10 micrometers. In other embodiments, the spacing between wells is at least 5 micrometers, at least 10 micrometers, at least 15 micrometers, at least 20 micrometers, at least 25 micrometers, at least 30 micrometers, at least 35 micrometers, at least 40 micrometers, at least 45 micrometers, at least 50 micrometers, at least 55 micrometers, at least 60 micrometers, at least 65 micrometers, at least 70 micrometers, or at least 75 micrometers. The microwell spacing can be at most 75 micrometers, at most 70 micrometers, at most 65 micrometers, at most 60 micrometers, at most 55 micrometers, at most 50 micrometers, at most 45 micrometers, at most 40 micrometers, at most 35 micrometers, at most 30 micrometers, at most 25 micrometers, at most 20 micrometers, at most 15 micrometers, at most 10 micrometers, at most 5 micrometers. The microwell spacing can be about 55 micrometers. The microwell spacing may fall within any range bounded by any of these values (e.g. from about 18 micrometers to about 72 micrometers). 
     The microwell array may comprise surface features between the microwells that are designed to help guide cells and solid supports into the wells and/or prevent them from settling on the surfaces between wells. Examples of suitable surface features can include, but are not limited to, domed, ridged, or peaked surface features that encircle the wells or straddle the surface between wells. 
     The total number of wells in the microwell array can be determined by the pattern and spacing of the wells and the overall dimensions of the array. The number of microwells in the array can range from about 96 to about 5,000,000 or more. The number of microwells in the array can be at least 96, at least 384, at least 1,536, at least 5,000, at least 10,000, at least 25,000, at least 50,000, at least 75,000, at least 100,000, at least 500,000, at least 1,000,000, or at least 5,000,000. The number of microwells in the array can be at most 5,000,000, at most 1,000,000, at most 75,000, at most 50,000, at most 25,000, at most 10,000, at most 5,000, at most 1,536, at most 384, or at most 96 wells. The number of microwells in the array can be about 96, 384, and/or 1536. The number of microwells can be about 150,000. The number of microwells in the array may fall within any range bounded by any of these values (e.g. from about 100 to 325,000). 
     Microwell arrays may be fabricated using any of a number of fabrication techniques. Examples of fabrication methods that may be used include, but are not limited to, bulk micromachining techniques such as photolithography and wet chemical etching, plasma etching, or deep reactive ion etching; micro-molding and micro-embossing; laser micromachining; 3D printing or other direct write fabrication processes using curable materials; and similar techniques. 
     Microwell arrays can be fabricated from any of a number of substrate materials. The choice of material can depend on the choice of fabrication technique, and vice versa. Examples of suitable materials can include, but are not limited to, silicon, fused-silica, glass, polymers (e.g. agarose, gelatin, hydrogels, polydimethylsiloxane (PDMS; elastomer), polymethylmethacrylate (PMMA), polycarbonate (PC), polypropylene (PP), polyethylene (PE), high density polyethylene (HDPE), polyimide, cyclic olefin polymers (COP), cyclic olefin copolymers (COC), polyethylene terephthalate (PET), epoxy resins, thiol-ene based resins, metals or metal films (e.g. aluminum, stainless steel, copper, nickel, chromium, and titanium), and the like. In some instances, the microwell comprises optical adhesive. In some instances, the microwell is made out of optical adhesive. In some instances, the microwell array comprises and/or is made out of PDMS. In some instances, the microwell is made of plastic. A hydrophilic material can be desirable for fabrication of the microwell arrays (e.g. to enhance wettability and minimize non-specific binding of cells and other biological material). Hydrophobic materials that can be treated or coated (e.g. by oxygen plasma treatment, or grafting of a polyethylene oxide surface layer) can also be used. The use of porous, hydrophilic materials for the fabrication of the microwell array may be desirable in order to facilitate capillary wicking/venting of entrapped air bubbles in the device. The microwell array can be fabricated from a single material. The microwell array may comprise two or more different materials that have been bonded together or mechanically joined. 
     Microwell arrays can be fabricated using substrates of any of a variety of sizes and shapes. For example, the shape (or footprint) of the substrate within which microwells are fabricated may be square, rectangular, circular, or irregular in shape. The footprint of the microwell array substrate can be similar to that of a microtiter plate. The footprint of the microwell array substrate can be similar to that of standard microscope slides, e.g. about 75 mm long×25 mm wide (about 3″ long×1″ wide), or about 75 mm long×50 mm wide (about 3″ long×2″ wide). The thickness of the substrate within which the microwells are fabricated may range from about 0.1 mm thick to about 10 mm thick, or more. The thickness of the microwell array substrate may be at least 0.1 mm thick, at least 0.5 mm thick, at least 1 mm thick, at least 2 mm thick, at least 3 mm thick, at least 4 mm thick, at least 5 mm thick, at least 6 mm thick, at least 7 mm thick, at least 8 mm thick, at least 9 mm thick, or at least 10 mm thick. The thickness of the microwell array substrate may be at most 10 mm thick, at most 9 mm thick, at most 8 mm thick, at most 7 mm thick, at most 6 mm thick, at most 5 mm thick, at most 4 mm thick, at most 3 mm thick, at most 2 mm thick, at most 1 mm thick, at most 0.5 mm thick, or at most 0.1 mm thick. The thickness of the microwell array substrate can be about 1 mm thick. The thickness of the microwell array substrate may be any value within these ranges, for example, the thickness of the microwell array substrate may be between about 0.2 mm and about 9.5 mm. The thickness of the microwell array substrate may be uniform. 
     A variety of surface treatments and surface modification techniques may be used to alter the properties of microwell array surfaces. Examples can include, but are not limited to, oxygen plasma treatments to render hydrophobic material surfaces more hydrophilic, the use of wet or dry etching techniques to smooth (or roughen) glass and silicon surfaces, adsorption or grafting of polyethylene oxide or other polymer layers (such as pluronic), or bovine serum albumin to substrate surfaces to render them more hydrophilic and less prone to non-specific adsorption of biomolecules and cells, the use of silane reactions to graft chemically-reactive functional groups to otherwise inert silicon and glass surfaces, etc. Photodeprotection techniques can be used to selectively activate chemically-reactive functional groups at specific locations in the array structure, for example, the selective addition or activation of chemically-reactive functional groups such as primary amines or carboxyl groups on the inner walls of the microwells may be used to covalently couple oligonucleotide probes, peptides, proteins, or other biomolecules to the walls of the microwells. The choice of surface treatment or surface modification utilized can depend both or either on the type of surface property that is desired and on the type of material from which the microwell array is made. 
     The openings of microwells can be sealed, for example, during cell lysis steps to prevent cross hybridization of target nucleic acid between adjacent microwells. A microwell (or array of microwells) may be sealed or capped using, for example, a flexible membrane or sheet of solid material (i.e. a plate or platten) that clamps against the surface of the microwell array substrate, or a suitable bead, where the diameter of the bead is larger than the diameter of the microwell. 
     A seal formed using a flexible membrane or sheet of solid material can comprise, for example, inorganic nanopore membranes (e.g., aluminum oxides), dialysis membranes, glass slides, coverslips, elastomeric films (e.g. PDMS), or hydrophilic polymer films (e.g., a polymer film coated with a thin film of agarose that has been hydrated with lysis buffer). 
     Solid supports (e.g., beads) used for capping the microwells may comprise any of the solid supports (e.g., beads) of the disclosure. In some instances, the solid supports are cross-linked dextran beads (e.g., Sephadex). Cross-linked dextran can range from about 10 micrometers to about 80 micrometers. The cross-linked dextran beads used for capping can be from 20 micrometers to about 50 micrometers. In some embodiments, the beads may be at least about 10, 20, 30, 40, 50, 60, 70, 80 or 90% larger than the diameter of the microwells. The beads used for capping may be at most about 10, 20, 30, 40, 50, 60, 70, 80 or 90% larger than the diameter of the microwells. 
     The seal or cap may allow buffer to pass into and out of the microwell, while preventing macromolecules (e.g., nucleic acids) from migrating out of the well. A macromolecule of at least about 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 12, 13, 14, 15, 16, 17, 18, 19, or 20 or more nucleotides may be blocked from migrating into or out of the microwell by the seal or cap. A macromolecule of at most about 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 12, 13, 14, 15, 16, 17, 18, 19, or 20 or more nucleotides may be blocked from migrating into or out of the microwell by the seal or cap. 
     Solid supports (e.g., beads) may be distributed among a substrate. Solid supports (e.g., beads) can be distributed among wells of the substrate, removed from the wells of the substrate, or otherwise transported through a device comprising one or more microwell arrays by means of centrifugation or other non-magnetic means. A microwell of a substrate can be pre-loaded with a solid support. A microwell of a substrate can hold at least 1, 2, 3, 4, or 5, or more solid supports. A microwell of a substrate can hold at most 1, 2, 3, 4, or 5 or more solid supports. In some instances, a microwell of a substrate can hold one solid support. 
     Individual cells and beads may be compartmentalized using alternatives to microwells, for example, a single solid support and single cell could be confined within a single droplet in an emulsion (e.g. in a droplet digital microfluidic system). 
     Cells could potentially be confined within porous beads that themselves comprise the plurality of tethered stochastic barcodes. Individual cells and solid supports may be compartmentalized in any type of container, microcontainer, reaction chamber, reaction vessel, or the like. 
     Single cell combinatorial barcoding or may be performed without the use of microwells. Single cell, combinatorial barcoding assays may be performed without the use of any physical container. For example, combinatorial barcoding without a physical container can be performed by embedding cells and beads in close proximity to each other within a polymer layer or gel layer to create a diffusional barrier between different cell/bead pairs. In another example, combinatorial barcoding without a physical container can be performed in situ, in vivo, on an intact solid tissue, on an intact cell, and/or subcellularly. 
     Microwell arrays can be a consumable component of the assay system. Microwell arrays may be reusable. Microwell arrays can be configured for use as a stand-alone device for performing assays manually, or they may be configured to comprise a fixed or removable component of an instrument system that provides for full or partial automation of the assay procedure. In some embodiments of the disclosed methods, the bead-based libraries of stochastic barcodes can be deposited in the wells of the microwell array as part of the assay procedure. In some embodiments, the beads may be pre-loaded into the wells of the microwell array and provided to the user as part of, for example, a kit for performing stochastic barcoding and digital counting of nucleic acid targets. 
     In some embodiments, two mated microwell arrays are provided, one pre-loaded with beads which are held in place by a first magnet, and the other for use by the user in loading individual cells. Following distribution of cells into the second microwell array, the two arrays may be placed face-to-face and the first magnet removed while a second magnet is used to draw the beads from the first array down into the corresponding microwells of the second array, thereby ensuring that the beads rest above the cells in the second microwell array and thus minimizing diffusional loss of target molecules following cell lysis, while maximizing efficient attachment of target molecules to the stochastic barcodes on the bead. 
     Microwell arrays of the disclosure can be pre-loaded with solid supports (e.g., beads). Each well of a microwell array can comprise a single solid support. At least 10, 20, 30, 40, 50, 60, 70, 80, 90, or 100% of the wells in a microwell array can be pre-loaded with a single solid support. At most 10, 20, 30, 40, 50, 60, 70, 80, 90, or 100% of the wells in a microwell array can be pre-loaded with a single solid support. The solid support can comprise stochastic barcodes and/or combinatorial barcodes of the disclosure. Cell labels of stochastic barcodes on different solid supports can be different. Cell labels of stochastic barcodes on the same solid support can be the same. 
     While various aspects and embodiments have been disclosed herein, other aspects and embodiments will be apparent to those skilled in the art. The various aspects and embodiments disclosed herein are for purposes of illustration and are not intended to be limiting, with the true scope and spirit being indicated by the following claims. 
     One skilled in the art will appreciate that, for this and other processes and methods disclosed herein, the functions performed in the processes and methods can be implemented in differing order. Furthermore, the outlined steps and operations are only provided as examples, and some of the steps and operations can be optional, combined into fewer steps and operations, or expanded into additional steps and operations without detracting from the essence of the disclosed embodiments. 
     With respect to the use of substantially any plural and/or singular terms herein, those having skill in the art can translate from the plural to the singular and/or from the singular to the plural as is appropriate to the context and/or application. The various singular/plural permutations may be expressly set forth herein for sake of clarity. 
     It will be understood by those within the art that, in general, terms used herein, and especially in the appended claims (e.g., bodies of the appended claims) are generally intended as “open” terms (e.g., the term “including” should be interpreted as “including but not limited to,” the term “having” should be interpreted as “having at least,” the term “includes” should be interpreted as “includes but is not limited to,” etc.). It will be further understood by those within the art that if a specific number of an introduced claim recitation is intended, such an intent will be explicitly recited in the claim, and in the absence of such recitation no such intent is present. For example, as an aid to understanding, the following appended claims may contain usage of the introductory phrases “at least one” and “one or more” to introduce claim recitations. However, the use of such phrases should not be construed to imply that the introduction of a claim recitation by the indefinite articles “a” or “an” limits any particular claim containing such introduced claim recitation to embodiments containing only one such recitation, even when the same claim includes the introductory phrases “one or more” or “at least one” and indefinite articles such as “a” or “an” (e.g., “a” and/or “an” should be interpreted to mean “at least one” or “one or more”); the same holds true for the use of definite articles used to introduce claim recitations. In addition, even if a specific number of an introduced claim recitation is explicitly recited, those skilled in the art will recognize that such recitation should be interpreted to mean at least the recited number (e.g., the bare recitation of “two recitations,” without other modifiers, means at least two recitations, or two or more recitations). Furthermore, in those instances where a convention analogous to “at least one of A, B, and C, etc.” is used, in general such a construction is intended in the sense one having skill in the art would understand the convention (e.g., “a system having at least one of A, B, and C” would include but not be limited to systems that have A alone, B alone, C alone, A and B together, A and C together, B and C together, and/or A, B, and C together, etc.). In those instances where a convention analogous to “at least one of A, B, or C, etc.” is used, in general such a construction is intended in the sense one having skill in the art would understand the convention (e.g., “a system having at least one of A, B, or C” would include but not be limited to systems that have A alone, B alone, C alone, A and B together, A and C together, B and C together, and/or A, B, and C together, etc.). It will be further understood by those within the art that virtually any disjunctive word and/or phrase presenting two or more alternative terms, whether in the description, claims, or drawings, should be understood to contemplate the possibilities of including one of the terms, either of the terms, or both terms. For example, the phrase “A or B” will be understood to include the possibilities of “A” or “B” or “A and B.” 
     In addition, where features or aspects of the disclosure are described in terms of Markush groups, those skilled in the art will recognize that the disclosure is also thereby described in terms of any individual member or subgroup of members of the Markush group. 
     As will be understood by one skilled in the art, for any and all purposes, such as in terms of providing a written description, all ranges disclosed herein also encompass any and all possible subranges and combinations of subranges thereof. Any listed range can be easily recognized as sufficiently describing and enabling the same range being broken down into at least equal halves, thirds, quarters, fifths, tenths, etc. As a non-limiting example, each range discussed herein can be readily broken down into a lower third, middle third and upper third, etc. As will also be understood by one skilled in the art all language such as “up to,” “at least,” and the like include the number recited and refer to ranges which can be subsequently broken down into subranges as discussed above. Finally, as will be understood by one skilled in the art, a range includes each individual member. Thus, for example, a group having 1-3 cells refers to groups having 1, 2, or 3 cells. Similarly, a group having 1-5 cells refers to groups having 1, 2, 3, 4, or 5 cells, and so forth. 
     From the foregoing, it will be appreciated that various embodiments of the present disclosure have been described herein for purposes of illustration, and that various modifications may be made without departing from the scope and spirit of the present disclosure. Accordingly, the various embodiments disclosed herein are not intended to be limiting, with the true scope and spirit being indicated by the following claims.