ESR1 and breast cancer: Recently, due to the striking clinical trial results of the CDK4/6 inhibitors demonstrating substantial improvements in progression-free survival 4-8, these inhibitors have transited rapidly from preclinical studies to the clinical arena, and three of them (palbociclib 6, abemaciclib 9, 10 and ribociclib 11) have already been approved for the treatment of advanced, estrogen receptor (ER)-positive breast cancer patients.