PIK3CA and breast cancer: Activating mutations of the PIK3CA gene, seen in about 20% to 30% of HER2+ tumors and 30% to 35% of HR+ BCs [19,39,40], are associated with worse outcomes in clinical trials of HER2-targeted therapies than wild-type PIK3CA [19,40,41,42,43], particularly if the BC is also HR+ [42,43].