RB1 and breast cancer: However, although studies have detected Rb inactivation by hyperphosphorylation in breast cancer cells resistant to endocrine therapy and in patients’ tumors receiving adjuvant endocrine treatment (Thangavel et al., 2011; Malorni et al., 2016), large clinical trials couldn’t indicate a statistically relevant link between Rb and resistance to CDK4/6 inhibitors (Turner et al., 2019; Finn et al., 2020).