ESR1 and breast cancer: Most palbociclib initiators were between the ages of 45–64 (60.9%), previously used an aromatase inhibitor (62.5%), had a secondary malignancy/metastasis prior to the index date (87.4%), and had advanced stage ER+/HER2- breast cancer (93.5%) [18].Most patients were female, although 53 males (2.2%) were dispensed palbociclib.