PIK3CA and breast cancer: Given that components of the PI3K pathway, and specifically the PIK3CA gene, are frequently mutated in breast cancer (77% and 35%, respectively) [38] and that crosstalk between ER and PI3K signaling pathways has been established [17, 39], we asked whether PI3K inhibition could decrease proliferation of breast cancer cells in the context of resistance to aromatase inhibition.