Iressa inhibits both E2- and P-induced EB formation and epithelial outgrowth. Pubertal 4-week-old BALB/c were OVX, allowed to recover for 3 weeks, and then treated for 5 days with vehicle control (C), E2 ± Iressa (300 mg/kg), or P ± Iressa. A, Whole mounts showing effect of treatment on EB formation after 5 days of C, E2, E2+Iressa, P, or P+Iressa. Note the absence of EBs in Iressa-treated mammary glands. Scale bar, 2 mm. B, EB quantitation from C and treated whole mounts. Values represent the mean ± SEM number of EBs in an inguinal mammary gland (n = 4 mice). E2 and P increased the average EB number (*, P < .05), and this increase was inhibited by the addition of Iressa. C, Epithelial area quantitation from C and treated whole mounts. Values represent the mean ± SEM epithelial area in a pair of inguinal mammary glands (n = 4 mice). E2 and P increased the average epithelial area (*, P < .05), and this increase was inhibited by the addition of Iressa.