PIK3CA and breast cancer: Activating mutations in the PIK3 catalytic subunit alpha (PIK3CA) gene, which encodes the p110α-isoform of phosphatidylinositol 3-kinase (PI3K), occur in approximately 40% of patients with advanced HR+, HER2– breast cancer [6,7,8,9], and are associated with resistance to endocrine-based therapy and poor prognosis [6,10,11,12].