HER3 knock down decreases cell proliferation preferentially in PIK3CA mutant H1047R cells. (A) Cell proliferation assays were performed as described in Materials and Methods, with MCF-10A cells, knock in derivatives, HER3 knock down and controls, under growth media and assay media conditions. Data are shown as the percentage of cell proliferation under assay conditions (0.2 ng/mL EGF) relative to full growth media conditions at day 6. Error bars represent the standard error of the mean from triplicate samples and data are representative of three independent experiments. * is comparison between HER3 shRNA cells and their parental counterparts, while ** represents comparison between HER3 shRNA E545K versus HER3 shRNA H1047R cells. (B) Cell proliferation assays were performed as described in Materials and Methods, with MCF-7 (E545K), T47D (H1047R) cells, HER3 knock down and controls, with 1.25 nM 17-β-estradiol. Error bars represent the standard deviation from triplicate samples and data are representative of three independent experiments. *p < 0.01.