PIK3CA and breast cancer: The way BC is treated today has changed significantly as a consequence of the detailed definition of its molecular characteristics, the immunohistochemical receptors (e.g., ER, PR, HER2 (ERBB2), the Ki-67 cell proliferation index (MKI67)), genomic markers (e.g., BRCA1, BRCA2, and PIK3CA), and immunomarkers (e.g., tumor-infiltrating lymphocytes and PD-L1).