PIK3CA and breast cancer: Since Ibrahim and Juvekar et al. for the first time reported that blockage of PI3K/Akt pathway could result in HR deficiency by reducing BRCA1/2 expression and Rad51 foci formation in breast cancer [28, 47], a number of clinical trials and preclinical research combining PI3K inhibitors with PARP inhibitors were performed on HR proficient cancers [48–51].