CDK4 and neoplasm: High tumor CCNE1 expression was again associated with poor PFS among 391 patients treated with letrozole plus ribociclib in the MONALEESA-2 trial[41]; interestingly, this study also identified FGFR1 expression as a putative biomarker for CDK4/6 inhibitors (PFS of 22 months vs. not reached for patients with high vs. low FGFR1 expression, respectively).