ER subtype-specific induction of ER-dependent luciferase reporter gene expression in transiently transfected SKBR3 cells. Cells were cotransfected with the ER-responsive luciferase reporter plasmid pGudLuc7.ERE and the ERα or ERβ expression vector, incubated with the indicated concentration of (A) E2, (B) genistein, (C) Br-ERβ-041, or (D) PPT for 24 h, lysed and luciferase and total protein levels analyzed as described in Materials and Methods. Luciferase activity is expressed as a percent of maximum luciferase induction by 1 nM E2 and results represent the mean ± SD of triplicate determinations from each of three separate experiments after subtraction of the luciferase activity present in cells exposed to DMSO.