CDK4 and breast cancer: In the randomized phase II monarcHER trial, the combination therapy of CDK4/6 inhibitor abemaciclib plus trastuzumab, with or without fulvestrant, significantly improved PFS in women with hormone receptor-positive, HER2-positive, advanced BC compared to standard-of-care chemotherapy plus trastuzumab, and this combination regimen was safe and tolerable [221].