Expression of HRGβ1, erbB3 and erbB4 in tamoxifen-resistant MCF-7 cells. (a) RT-PCR analysis of basal HRGβ1 mRNA in DU145 prostate and tamoxifen-resistant MCF-7 (Tam-R) breast cancer cells. (b) Total erbB3 and erbB4 protein expression in Tam-R cells by western blotting and immunocytochemistry (original magnification, ×20). β-Actin protein expression was also assessed to demonstrate equivalent sample loading. Tamoxifen was present in Tam-R media. Data are representative of at least three separate experiments.