Effect of indigo, β-naphthoflavone (βNF), and 3,3',4,4',5-pentachlorobiphenyl (PCB126) on CYP1A, CYP1B1, CYP1C1, CYP1D1, and AHR gene expression in Xenopus tropicalis tadpoles. A) Chemical structures for indigo, βNF, and PCB126, B) relative CYP1 gene expression, C) distribution of developmental stages within the exposure groups, and D) relationship between log transformed gene expression and DRE number in the regions 0–20 kb upstream of the start codon in the four CYP1 genes. At 28 days post-fertilization tadpoles were exposed for 12 hours to 50 ppm of DMSO (carrier; light bars), or to 1 µM of indigo (grey bars), βNF (dark grey bars), or PCB126 (dark bars) including the carrier (at 26 °C). Calculations of relative expression were made using β-actin as reference gene and the mean values in the DMSO group were used as calibrators (E−ΔΔCt). Statistically significant differences in gene expression compared with the control were determined by one-way ANOVA followed by Dunnett’s post hoc test and are shown by stars (***p<0.001 and *p<0.05). Data representing 5–6 replicates per exposure group are shown as mean + SD.