The purpose of the phase II MonarcHER trial (NCT02675231) was to evaluate the efficacy of the combination of abemaciclib plus trastuzumab with (Arm A) or without (Arm B) fulvestrant in comparison to trastuzumab plus standard-of-care (SOC) chemotherapy (Arm C) in women with HR+/HER2+ locally advanced or metastatic BC after prior exposure to a minimum of two HER2-directed therapies for advanced disease [139,140]. Here, ERBB2 is linked to breast cancer.