ESR1 and breast carcinoma: Common coexisting mutations included ESR1 (41.6%), TP53 (25%), and PIK3CA (33.3%), all of which have been reported in endocrine resistant breast cancer.17 Interestingly, half the PIK3CA mutations were also present in the patients’ pretreatment metastatic tumor biopsies, which suggests these were possibly pre-existing truncal mutations (i.e. dominant mutations shared by subsequent clones).