PIK3CA and breast cancer: The PIK3CA mutations are relatively common in ER + breast cancers and are an early mutational event in the development of breast cancer.24 Previous reports have indicated that the prevalence of the PIK3CA mutations in primary and metastatic tumors is comparable and these mutations likely do not evolve under the selective pressure of current treatments.2 In this study, we analyzed the most common PIK3CA mutations in addition to the ESR1 mutations.