CDK4 and breast cancer: Several CDK4/6 inhibitors’ resistance mechanisms have been characterized in BC cells, including the loss of or mutations in the retinoblastoma (RB1) gene, the alteration of CDK4/6 and CDK2 signaling, and the activation of growth signaling pathways (Klein et al. 2018; McCartney et al. 2019; Pandey et al. 2019; Portman et al. 2018).