CYP19A1 and breast cancer: A number of CDK4/6 kinase inhibitors such as PD 0332991 [17] (Palbociclib), LY-2835219 (Abemaciclib), and LEE011 (Ribociclib) [18] in combination with an aromatase inhibitor have been approved by the FDA for treating patients with hormone receptor-positive and HER2-negative BC.