However, current highly-specific and potent CDK4/6 inhibitors, such as palbociclib (PD0332991), ribociclib (LEE011), and abemaciclib (LY2835219), have shown promising efficacy in patients, especially with RB-intact estrogen receptor (ER)-positive and HER2-negative breast cancer [10,11]. Here, ESR1 is linked to breast cancer.