Time-dependent (A) and stage-dependent (B) variations in basal expression of CYP1A, CYP1B1, CYP1C1, CYP1D, AHR, ARNT2, and β-actin in developing Xenopus tropicalis tadpoles. Time-dependent variations in basal expression were studied in unexposed tadpoles collected from 1 day to 16 days post-fertilization (dpf). Data are shown as mean + SD. N=4, where the number of tadpoles per replicate was five at 1–8 dpf and one at 16 dpf. Variations with developmental stage were studied in a group of 13 unexposed tadpoles sampled at 28 dpf, where each star represents data from one tadpole. Stages were determined according to Nieuwkoop and Faber (1994). Expression was calculated by E−Ct.