NR4A1 and breast cancer: The last generation of CDK4/6 inhibitors (CDKi) includes palbociclib (PD 0332991), ribociclib (LEE011), and abemaciclib (LY2835219), which are currently approved for the treatment of advanced and/or metastatic hormone receptor-positive/HER2-neu-negative (HR+/HER2−) breast cancer in combination with hormonal therapy.