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Author Manuscript
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Published in final edited form as:
Sci China Chem. 2010 May ; 53(5): 1010–1016. doi:10.1007/s11426-010-0142-8.

Enhancing the response of CALUX and CAFLUX cell bioassays
for quantitative detection of dioxin-like compounds
Bin ZHAO1,2,*, David S. BASTON3, Elaine KHAN4, Claudio SORRENTINO5, and Michael S.
DENISON2
1 State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center for EcoEnvironmental Sciences, Chinese Academy of Sciences, Beijing 100085, China
2 Department of Environmental Toxicology, University of California, Davis, California 95616, USA
3 Laboratorium voor Analytische en Milieu Chemie, Vrije Universiteit Brussel, B-1050 Brussels,

Belgium

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4 Pesticide Epidemiology Section, Pesticide and Environmental Toxicology Branch, Office ofEnvironmental Health Hazard Assessment, Sacramento, California 94612, USA
5 Human and Ecological Risk Office, Brownfields and Environmental Restoration Program,

Department of Toxic Substances Control, Sacramento, California 95812, USA

Abstract

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Reporter genes produce a protein product in transfected cells that can be easily measured in intact
or lysed cells and they have been extensively used in numerous basic and applied research
applications. Over the past 10 years, reporter gene assays have been widely accepted and used foranalysis of 2,3,7,8-tetrachlorodibenzo-p-dioxin and related dioxin-like compounds in various
types of matrices, such as biological, environmental, food and feed samples, given that highresolution instrumental analysis techniques are impractical for large-scale screening analysis. The
most sensitive cell-based reporter gene bioassay systems developed are the mechanism-based
CALUX (Chemically Activated Luciferase Expression) and CAFLUX (Chemically ActivatedFluorescent Expression) bioassays, which utilize recombinant cell lines containing stablytransfected dioxin (AhR)-responsive firefly luciferase or enhanced green fluorescent protein(EGFP) reporter genes, respectively. While the current CALUX and CAFLUX bioassays are very
sensitive, increasing their lower limit of sensitivity, magnitude of response and dynamic range forchemical detection would significantly increase their utility, particularly for those samples that
contain low levels of dioxin-like HAHs (i.e., serum). In this study, we report that the addition ofmodulators of cell signaling pathways or modification of cell culture conditions results insignificant improvement in the magnitude and overall responsiveness of the existing CALUX and
CAFLUX cell bioassays.

Keywords
dioxin; reporter gene; bioassay; CALUX; CAFLUX*Corresponding author (binzhao@rcees.ac.cn).

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1 Introduction
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Proper epidemiological, risk assessment and exposure analyses of 2,3,7,8tetrachlorodibenzo-p-dioxin (TCDD, dioxin) and related dioxin-like halogenated aromatichydrocarbons (HAHs) require accurate measurements of these chemicals both in the species
of interest and in various exposure matrices (i.e., biological, environmental, food and feed)[1–4]. While high-resolution instrumental analysis techniques are established for thesechemicals, current procedures are impractical for large-scale screening analysis.
Accordingly, numerous in vitro and cell-based bioanalytical methods have been developed
for the detection and relative quantitation of these chemicals in extracts from a variety ofmatrices and the majority of these systems take the advantage of one or more aspects of theaction mechanism of these chemicals (i.e., their ability to activate the Ah receptor (AhR)
and/or AhR-dependent gene expression) [1–5]. The most sensitive cell-based bioassay
systems developed are the CALUX (Chemically Activated Luciferase Expression) and
CAFLUX (Chemically Activated Fluorescent Expression) bioassays, which utilize
recombinant cell lines that contain stably transfected dioxin (AhR)-responsive firefly
luciferase or enhanced green fluorescent protein (EGFP) reporter genes, respectively [2,3].
Treatment of these cells with TCDD and related HAHs, other AhR ligands or extracts
containing AhR agonists, results in induction of reporter gene expression in a time-, dose-,
AhR-, and chemical-specific manner and the level of reporter gene expression directly
correlates with the total concentration of TCDD-like AhR inducers (agonists) present in thesample [3,4,6,7]. Although these systems use the same AhR-dependent mechanism to induce
reporter gene expression, differences in the characteristics of the respective reporter geneproducts result in bioassay systems with distinct advantages and disadvantages [2,3]. While
the firefly luciferase reporter gene in the CALUX bioassay system has a very high degree ofsensitivity and response, primarily due to enzymatic signal amplification, it also has
limitations with respect to repeated measurement, cost and rapidity for high-throughput
screening analysis. By contrast, measurement of EGFP reporter gene activity is more rapid,
cost effective, and it is amenable to high throughput and repeated analysis and the induction
response could be measured in “real time” [3]. However, as the EGFP output signal is
directly proportional to the number of EGFP molecules (i.e., it is not an amplified response
like luciferase) the reporter signal develops significantly slower than that of luciferase, but
because EGFP is extremely stable (T1/2 > 23 h), much higher reporter gene signal output isobtained with extended time of incubation. While the current CALUX and CAFLUX
bioassays are very sensitive, increasing their lower limit of sensitivity as well as magnitude
of response and dynamic range for chemical detection would significantly increase theirutility, particularly with samples that contain low levels of dioxin-like HAHs or where there
is a limited sample size. Recent reports of enhancement of AhR and AhR-dependent signaltransduction by chemical treatment as well as increased luciferase and EGFP reporter geneactivity by modulating cell culture conditions provide several ways to improve thesensitivity, signal-to-noise ratio, detection limits and responsiveness of current CALUX and
CAFLUX bioassays [6,8–11]. Here we report the results of studies examining the effect ofmodulators of cell signaling pathways (i.e., dexamethasone (DEX) and phorbol-12myristate-13-acetate (PMA)) as well as alterations in cell culture conditions on themagnitude and overall responsiveness of the CALUX and CAFLUX cell bioassays.

2 Materials and methods
2.1 Chemicals
TCDD was from Dr. Steven Safe (Texas A&M University, USA), dexamethasone from
Sigma Chemical Company (St. Louis, MO, USA) and phorbol-12-myristate-13-acetate from
CalbioChem (LaJolla, CA, USA).

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2.2 Preparation of sediment sample extracts for CALUX and CAFLUX bioassays

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Dry, crushed sediment samples (2 g) were extracted three times with 10 mL of toluene each.
All extracts were filtered and composited with a 10 mL toluene rinse of the filter column.
The extract was reduced to near dryness and cleaned up and fractionated on a combinationof an acid silica column and 1% carbon/celite fractionation column [12], with the PCB
fraction eluted first followed by the Dioxin/Furan fraction. Both fractions were reduced tonear dryness and resuspended in 4 mL of hexane and stored in the dark until analyzed. For
bioassay tested in this study, aliquots of each sample extract (Dioxin/Furan fraction) weredried and resuspended in a small volume of DMSO and tested in the CALUX/CAFLUX
bioassays as described.
2.3 Cell culture, chemical treatment, and CALUX/CAFLUX cell bioassays

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Recombinant rat (H4L1.1c4, H4G1.1c2), mouse (H1L1.1c2, H1L6.1c2 and H1G1.1c3),
human (HG2L6.1c3) hepatoma cells and guinea pig intestinal adenocarcinoma (G16L1.1c8)
were grown and maintained as previously described [2,13,14]. H4L1.1c4, H1L1.1c2 andG16L1.1c8 cells contain the stably integrated DRE-driven firefly luciferase reporter plasmid
pGudLuc1.1 [13] and H1L6.1c2 and HG2L6.1c3 cells contain pGudLuc6.1 [14], while
H4G1.1c2 and H1G1.1c3 contain the stably integrated DRE-driven EGFP reporter plasmid
pGreen1.1 [6]. Each of these recombinant cell lines responds to TCDD and related AhR
agonists with the induction of luciferase/EGFP reporter gene activity in a time-, dose-,
ligand-dependent manner as previously described [3,4,6,7]. Cells were plated into white,
clear-bottomed 96-well tissue culture dishes (for CALUX) or black, clear-bottomed 96-welltissue culture dishes (for CAFLUX) at 75000 cells per well and allowed to attach for 24 h.
Cells were incubated with the carrier solvent DMSO (1% final solvent concentration),
TCDD at indicated concentrations or the indicated treatment (e.g., sediment extracts) for
either 4 h (H4L1.1c4, H1L1.1c2 and G16L1.1c8 cells) or 24 h (H1L6.1c2, HG2L6.1c3,
H4G1.1c2 and H1G1.1c3 cells) at the specified temperature. For luciferase measurement,
sample wells were washed twice with phosphate-buffered saline, followed by addition ofcell lysis buffer (Promega) and shaking of the plates for 20 min at room temperature toallow complete cell lysis. Measurement of luciferase activity in each well was carried out
using an Anthos Lucy2 (Durham, NC) microplate luminometer with automatic injection ofPromega stabilized luciferase reagent. EGFP fluorescence in intact cells was measured
directly in 96 well plates without media removal using a Tecan GENios microplate
fluorometer with excitation and emission wavelengths of 485 and 515 nm, respectively.
Reporter gene activity in each well was expressed either as the original relative light units(RLUs) for the CALUX bioassay, relative fluorescence units (RFUs) for the CAFLUX
bioassay or values were normalized to the maximal activity induced by 1 nM TCDD.

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3 Results and discussion
3.1 The effect of bioassay incubation temperature on the inducibility and responsivenessof the CALUX and CAFLUX cell bioassays
In previous studies using the mouse hepatoma CAFLUX bioassay we demonstrated that thetissue culture incubation temperature during the induction period was critical in maximizing
the overall EGFP activity obtained by a defined amount of AhR agonist (TCDD) at 33 °C,
resulting in several fold greater EGFP activity than cells grown at 37 °C [6]. This increase
was presumed to be due to an increase in EGFP activity than from an increase in gene
expression. Although the firefly luciferase used in reporter gene studies has been reported tobe thermo labile at 37 °C, its relative activity in cells grown at other temperatures has notbeen determined [15]. To determine the effect of incubation temperature on the overall
activity of EGFP and luciferase, DMSO- and TCDD-treated mouse hepatoma CALUX and
CAFLUX cell lines (H1G1.1c3 and H1L6.1c2 cells, respectively) were incubated for 24 h at
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various temperatures (26 °C, 30 °C, 33 °C or 37 °C) followed by analysis of reporter geneactivity. The results of dose response studies for each reporter gene at different temperaturesare shown in Figure 1 (upper panel). As is readily apparent, cells incubated at 33 °C
exhibited significantly more EGFP activity than those at 37 °C, 30 °C or 26 °C. The
fluorescence of EGFP at 33 °C was 2–3 fold greater than that observed in cells incubated at
37 °C at all TCDD concentrations, and it improved the level of reporter gene activity at theminimal detection limit of 1 pM (compare 323 ± 31 RFUs at 33 °C to 126 ± 42 RFUs at 37°C). Similarly, a 5–10 fold greater amount of luciferase activity was obtained from cells that
had been incubated for 24 h at 30 °C or 33 °C than that obtained from cells incubated for 24
h at 37 °C or 26 °C (Figure 1, lower panel).
Luciferase activity in cells treated with 1 pM TCDD for 24 h at 33 °C was significantly
higher than that observed from cells incubated at 37 °C. In additional studies, we showedthat the observed temperature-dependence of reporter gene activity results from an effect on
the overall activity of the reporter gene itself, rather than an effect on the AhR signaling
pathway (data not shown). Although the underlying cause of the temperature dependentenhancement effect remains to be established, given previous studies on GFP and luciferase
reporter genes and the expected body temperature of the organism from which the reporter
genes were obtained, it is likely due to the differences in the folding and/or stability of these
proteins.

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We extended the above studies to investigate whether reduced temperature (33 °C) would
improve CALUX and CAFLUX analysis when applied to environmental sample extracts.
Sediment samples were collected and cleaned up in the lab following the standard
procedures to remove PAHs and other AhR agonists from the desired HAHs (i.e., dioxinlike PCDDs, PCDFs and PCBs). The TCDD standard was run in parallel with environmental
samples as the positive control and for comparative purpose. The results of TCDD dose
response studies for luciferase (CALUX assay) at 37 °C and 33 °C are shown in Figure 2(a)and consistent with previous results. Cells incubated at 33 °C exhibited significantly moreluciferase activity than that at 37 °C. The luciferase activity at 33 °C was 5–15 fold greaterthan that observed in cells incubated at 37 °C at all TCDD concentrations, and it increased
the amount of luciferase activity at the minimal detection limit of 0.45 pg TCDD/mL(compare 46 ± 19 relative light units (RLUs) at 33 °C to 1 ± 1 RLUs at 37 °C). The
luciferase activity at 33 °C was similarly 5–15 fold greater than that observed in cells
incubated at 37 °C when incubated with HAH containing sediment extracts (Figure 2(b)).
The amount of dioxin or dioxin-like compounds contained in extracts from less than 1 μg of
the sediment was not detectable (i.e., producing less than 10 RLU over the solvent control)in the standard CALUX assay temperature (37 °C), while they were significantly above thebackground activity when incubated at 33 °C given the significantly higher luciferase
activity (up to 300 RLU). This small temperature change (from 37 °C to 33 °C) could
significantly increase the bioassay’s detection of TCDD-like compounds in environmental
samples using the CALUX bioassay by up to 2 orders of magnitude. Similar results wereobtained using the CAFLUX assay, where about a 2-fold greater amount of fluorescence
activity from GFP was obtained from cells that had been incubated at 33 °C compared tothose incubated at 37 °C with either TCDD standard or environment extract (Figure 2(c) and(d)). GFP fluorescence activity in cells treated with 0.45 pg TCDD/ml for 24 h at 33 °C wassignificantly higher than that observed from cells incubated at 37 °C, and the overall activity
at 33 °C was significantly above the background, while GFP activity at 37 °C wasindistinguishable from the background (Figure 2(c)). The GFP fluorescence activity induced
by TCDD at 33 °C was ~2 fold greater than that observed in cells incubated at 37 °C at allconcentrations of sediment extracts (Figure 2(d)). We observed similar temperaturedependent differences in reporter gene activity (with optimal activity at 33 °C) using rat andhuman liver and guinea pig intestinal cell lines (H4L1.1c4, H4G1.1c2; HG2L6.1c3;
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G16L1.1c8 cells) containing a stably transfected AhR- responsive luciferase or EGFP
reporter gene (Table 1).

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A similar enhancement of estrogen receptor-responsive luciferase reporter gene activity at
33 °C as compared to 37 °C (Table 1) confirms that the enhancing effect is not an AhRspecific response. These results demonstrate that our observed temperature-dependence ofreporter gene activity is due to an effect on the reporter gene itself, rather than an effect on
the AhR from an individual species and that the current CALUX and CAFLUX bioassays
are carried out at suboptimal temperatures necessary for production of maximal reporter
gene activity. In those situations where optimal reporter gene activity in transfectedmammalian cells is desired, cells are incubated at lower temperatures (i.e., 33 °C) during thechemical incubation period. This simple modification of gene induction protocols could leadto substantial increase in measurable reporter gene activity and therefore could significantly
increase the overall response, minimal detection limit and range of bioassay response fordioxin and dioxin-like compounds in sample extracts.
3.2 The effect of dexamethasone (DEX) on the inducibility and responsiveness of the
CALUX cell bioassay

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The ability of glucorticoid receptor (GR) agonists as DEX to enhance AhR-dependent
induction in several cell lines and the rat hepatoma CALUX (pGudLuc1.1) cell line has beenpreviously reported and is suggested to result from increased expression of the AhR [8,9].
Therefore, we considered that the addition of DEX to the standard bioassay incubation
might provide an approach to improve these bioassays by increasing their sensitivity andmagnitude of response. Accordingly, the effect of DEX on the induction of reporter gene
expression by TCDD was examined in rat, mouse and guinea pig CALUX cell lines (Figure3). While exposure of the rat hepatoma and guinea pig intestinal adenocarcinoma CALUX
cell bioassays to DEX resulted in an increase in the magnitude of TCDD induced luciferase
activity, DEX had no effect on TCDD-dependent luciferase gene induction in the mouse
CALUX cell lines. The ability of the GR antagonist RU486 to inhibit the DEX enhancementof TCDD induction but not affect TCDD induction in the absence of DEX in the rat celllines indicates that the enhancement by DEX was independent of the AhR and involved the
GR (data not shown). DEX alone induced luciferase/EGFP activity in the rat CALUX/CAFLUX cell lines and this induction was confirmed to be dependent on the GR as RU486
also inhibited this effect (data not shown). The inability of DEX to affect TCDD inductionin the mouse liver cell lines, which are known to contain GRs, demonstrated the speciesspecificity of the response. The ability of DEX to enhance AhR-dependent reporter gene
expression in the rat and guinea pig CALUX cell lines indicated it is a possible candidate forthe enhancement of the bioassay system for detection and analysis of environmental
samples. By contrast, an increased response in rat and guinea pig CALUX cell lines bycomplex mixtures of chemicals that contain both AhR and GR agonists could lead tooverestimation of the potency and/or a false positive result. Accordingly, the mouse
CALUX bioassay would be expected to result in a more accurate potency determination
when GR agonists are present in an unknown sample extract.
3.3 The effect of phorbol-12-myristate-13-acetate (PMA) on the inducibility andresponsiveness of the CALUX cell bioassay
The AhR and ARNT have been shown to be phosphoproteins and the role ofphosphorylation in the regulation of AhR signaling and species- and tissue-specificresponsiveness to TCDD and other ligands is being examined. The role of protein kinase C(PKC) in the modulation of AhR responsiveness is of particular interest since activation of
PKC by PMA was reported to augment TCDD-inducible AhR- and DRE-dependent gene
expression by 2- to 3-fold in rats and mice [10,11]. Thus, we tested whether the inclusion of

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PMA into the standard CALUX/CAFLUX bioassay incubation protocol might improvebioassay detection and response. In contrast to the DEX experiments, exposure of the mouseor rat hepatoma CALUX cells to PMA alone had no significant effect, but when added
together with TCDD or BNF (another AhR agonist), it significantly enhanced dosedependent luciferase gene induction by 3–4 folds (Figure 4). While these results
demonstrated that the addition of PMA to the inducing solution can increase the magnitudeof the induction response, comparisons, it did not increase the minimal detection limits of
the assay. The PMA enhancement of AhR signaling is transient, occurring within eight
hours after PMA treatment, and decreasing thereafter as PKC is feedback inhibited (data notshown). Thus, while PMA could be used to increase the responsiveness of the CALU
bioassay, the transient nature of the enhancement restricts the window of time that PMA
could be used to improve detection and quantitation of dioxin-like chemicals by the CALUX
bioassay.
3.4 Demonstration of superinduction of CALUX luciferase activity by environmental
samples

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Given the ability to enhance the CALUX response by addition of exogenous chemicals, wefound sample extracts that produce a similar superinduction phenomenon. During our
extensive analyses of sediment and soil samples by CALUX, we found numerous samples
that induced CALUX luciferase gene expression to levels of 2–3 times greater than thatinduced by a maximal inducing concentration of TCDD. An example is shown in Figure 5.
While one sediment sample extract induced CALUX luciferase activity in mouse H1L1.1c2
cells to the same maximal activity as that of TCDD (Figure 5, sample 1), another sample
extract prepared at the same time resulted in superinduction of CALUX luciferase reporter
gene activity, still producing a full concentration-response curve (Figure 5, sample 2). The
superinduction phenomenon is not unique or an artifact of the AhR CALUX bioassay, as wehave observed similar superinduction responses using a stably transfected estrogen receptor(ER)-responsive luciferase reporter gene in human ovarian carcinoma cells (BG1) following
exposure to crude solvent extracts of plants (data not shown). It remains to be determined
what accounts for the superinduction response to the sediment extracts in the CALUX cellline. It was envisioned that the presence of chemicals in this sample extract could activate
cell signaling pathways to enhance AhR-dependent gene expression, CALUX plasmid
promoter activity or luciferase reporter gene activity, similar to effects shown in Figures 3
and 4. While the exact mechanism of superinduction from the environmental sample isunknown, further investigation may allow identification of responsible mechanisms for theenhancement effect that could be incorporated into the standard CALUX bioassay protocol
to significantly increase assay sensitivity and response.

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4 Conclusions
While the current CALUX and CAFLUX bioassays are very sensitive, increasing their lower
limit of sensitivity as well as magnitude of response and dynamic range for chemical
detection would significantly increase their utility, particularly for those samples that
contain low levels of dioxin-like HAHs or in which low sample volumes are available. In
this study we demonstrated that by the application of modulators of cell signaling pathways(i.e., dexamethasone (DEX) and phorbol-12-myristate-13-acetate (PMA)) as well as
alterations in cell culture conditions (bioassay incubation temperature), we could
significantly improve the sensitivity, signal-to-noise ratio, detection limits andresponsiveness of the CALUX and CAFLUX bioassays. Further studies of the biochemical
and molecular mechanisms may facilitate the integration of these and other changes into
protocols, which may improve the CALUX and CAFLUX bioassays for dioxins and related
dioxin-like chemicals as well as other AhR agonists.

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Acknowledgments
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The authors express their great thanks for the jointly support from the National Institutes of Environmental Health
Sciences Superfund Basic Research Grant (ES004699 (MSD)), the California Agricultural Experiment Station, theStartup Fund of 100 Talents Program of Chinese Academy of Sciences, the National Natural Science Foundation ofChina (20921063) and the National Basic Research Program (2010CB933500).

References

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Figure 1.

Temperature-dependent enhancement of TCDD-inducible reporter gene expression in mouse
CAFLUX (H1G1.1c3) and CALUX (H1L6.1c2) cell lines. H1L6.1c2 and H1G1.1c3 cells
were incubated at the indicated temperature with increasing concentrations of TCDD for 24
h. EGFP and luciferase activity were determined as described. Values are expressed as therelative light or fluorescence units (RLUs and RFUs, respectively) and represent the mean ±SD of triplicate determinations.

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Figure 2.

Temperature-dependent enhancement of reporter gene expression in mouse CALUX(H1L6.1c2) and CAFLUX (H1G1.1c3) cell lines by sediment extracts. H1L6.1c2 andH1G1.1c3 cells were incubated at the indicated temperature with either increasing
concentrations of TCDD standard ((a) and (c)) or a sediment sample extract ((b) and (d)) for
24 h. Luciferase and EGFP activity were determined as described. Values are expressed as
the relative light or fluorescence units (RLUs and RFUs, respectively) and represent themean ± SD of triplicate determinations.

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Figure 3.

Addition of dexamethasone (DEX) synergistically increases TCDD-dependent induction of
luciferase activity in the CALUX cell bioassay. H4L1.1c4, H1L1.1c2 and G16L1.1c8 cells
were incubated with 1 nM TCDD, 10 μM DEX or TCDD plus DEX for 4 h as indicated.
Luciferase activity was determined as described. Luciferase values are expressed as thepercentage of that induced by 1 nM TCDD and represent the mean ± SD of triplicate
determinations.

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Figure 4.

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Coincubation with PMA synergistically increases TCDD-dependent induction of luciferase
activity in the CALUX cell bioassay. Mouse CALUX (H1L1.1c2) cells were incubated withincreasing concentrations of TCDD or beta-naphthoflavone (BNF) in the absence orpresence of PMA (81 nM) for 4 h. Luciferase activity was determined as described under
Materials and methods. Luciferase values are expressed as the percentage of that induced by
1 nM TCDD and represent the mean ± SD of triplicate determinations.

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Figure 5.

Dose dependent luciferase reporter gene expression induced by sediment sample extracts inmouse CALUX (H1L6.1c2) cells. H1L6.1c2 cells were incubated with the indicatedequivalent concentration of sediment extract for 24 h. Luciferase activity was determined as
described under Materials and methods. Values are expressed as the percentage of thatinduced by 1 nM TCDD and represent the mean ± SD of triplicate determinations.

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a)

1±1

ETOH

1350 ± 18

BG1Luc4E2

3409 ± 86

H4G1.1c2

HG2L6.1c3

H1G1.1c2

1±1
19 ± 1

H1L1.1c2

44 ± 3100 ± 7

G16L1.1c8

H1L6.1c2

86 ± 1438 ± 32

estradiol

3309 ± 1512477 ± 365

245 ± 24

163 ± 271080 ± 97

310 ± 83187 ± 100

TCDD

1±1

ETOH

1266 ± 21

2698 ± 73

2±1

1±1

6±1

6±1

12 ± 137 °C DMSO

Reporter gene activity (RLU/RFU)
33 °C DMSO

H4L1.1c4

Cell linea)

59 ± 2

estradiol2123 ± 40

8188 ± 485

178 ± 133 ± 11139 ± 664 ± 21252 ± 62

TCDD

Rat (H4L1.1c4, H4G1.1c2), guinea pig (G16L1.1c8), mouse (H1L6.1c2, H1L1.1c2, H1G1.1c2) and human (HG2L6.1c3, BG1Luc4E2) cell lines.

ER luciferase

EGFP

AhR luciferase

Reporter

Summary of temperature-dependent reporter gene expression in a number of CALUX or CAFLUX cell lines

NIH-PA Author Manuscript

Table 1ZHAO et al.
Page 13

Sci China Chem. Author manuscript; available in PMC 2011 May 1.