ESR1 and breast cancer: In the clinic, three CDK4/6 inhibitors, namely, palbociclib (Fry et al., 2004), ribociclib (Infante et al., 2016), and abemaciclib (Patnaik et al., 2016), have been successfully used in combination with other endocrine therapy drugs for ER-positive and human epidermal growth factor receptor-2 (HER2)–negative advanced breast cancer treatment (Ribnikar et al., 2019); in addition, significant overall survival (OS) benefits have been confirmed at ESMO2019 conference.