CDK4 and breast cancer: Upregulation of anti-apoptotic proteins Bcl-2, Bcl-xl, and Mcl-1 in endocrine therapy-resistant (Williams et al., 2018), CDK4/6i-resistant (Whittle et al., 2020), and HER2 TKI-resistant (Eustace et al., 2018) breast cancer cell lines validates this theory.