COS-7 cells were co-transfected with expression plasmids for an AHR, X. laevis ARNT1, pGudLuc6.1 (reporter construct), and pRL-TK (transfection control construct) as described in Materials and Methods. Cells were treated with DMSO or the indicated concentrations of TCDD, and luciferase activity was measured after 18 hours. (A) Expression of AHRs determined by western blotting. (B) Transactivation activity of AHR1β, mouse AHRb-1, and chimeric frog AHR containing the mouse LBD. RLUs are given as a ratio of firefly to Renilla luciferase activity. (C) Fractional induction. To compare responsiveness more readily, luciferase activity at each TCDD concentration was normalized to the maximum response of each AHR. Values represent the mean ± standard error for 4-7 replicate analyses.