Within the group of kinase inhibitors, cyclin-dependent kinase 4/6 (CDK4/6) inhibitors, including Palbociclib and Abemaciclib, have been successfully used in the treatment of advanced estrogen receptor-positive (ER+) and human epidermal growth factor receptor 2 (HER-2)-negative breast cancers [15,16,17]. The gene discussed is ESR1; the disease is breast cancer.