In particular, the clinical consequences of PIK3CA mutations might vary according to the status of well-known molecular markers in breast cancer, namely ERα, progesterone receptor (PR), and ERBB2. Here, we examined the prognostic value of PIK3CA mutation status in a series of 452 patients with unilateral invasive primary breast cancer and known long-term outcome, taking ERα, PR, and ERBB2 status into account. Here, ESR1 is linked to breast cancer.