PIK3CA and breast cancer: The essential role of PI3K in the transcriptional regulation of Erbb2 was further supported by ChIP-qPCR analysis showing that overexpression of the mostly represented Pik3ca activating mutation in breast cancer, the Pik3caH1047R, in mammary IMEC-MYC cells enhances the transcriptional activity of both Erbb2 promoters (prom 1 and prom 2) and ENH1 and HGE (online supplemental figure 2E).