Indices of AhR activity in preconfluent and confluent MCF10A cells. A, MCF10A cells were harvested at approximately 70 and 100% confluence, and CYP1A1 mRNA levels were measured using a TaqMan Gene Expression Assay. Data are expressed as the mean ± S.E.M. of three independent experiments. B, MCF10A cells stably expressing an AhR-responsive luciferase reporter (pGudLuc1.1) were treated for 24 h with 0.1% DMSO or 30 nM TCDD before harvest at approximately 70 and 100% confluence for measurement of firefly luciferase activity and protein content. Data are expressed as mean relative luciferase activities (normalized to cellular protein content) ± S.D., three wells per treatment group. ***, p < 0.001 compared with preconfluent cells.