ERBB2 and breast cancer: The phase 2 MONARCH-1 (NCT02102490) trial is assessing the role of abemaciclib monotherapy in patients with previously treated, HR-positive, HER2-negative, advanced breast cancer, while MONARCH-3 (NCT02246621) is a randomized, double-blind, placebo-controlled trial of non-steroidal aromatase inhibitors with or without abemaciclib in women with advanced HR-positive, HER2-negative disease who have not had prior systemic therapy in this setting.