ESR1 and breast cancer: Sixteen ER+ MBC patients (15 HER2− and 1 HER2+) with a median age of 54.7 years old (range 32.6–74.8 years) who received palbociclib for a metastatic breast cancer diagnosis were identified from the cohort of 162 breast cancer patients enrolled in the study between February 2014 to April 2016, which preceded FDA approval for ribociclib and abemaciclib.