However, the PALOMA-2 and MONALEESA-2 trials evaluating CDK4/6i and letrozole versus letrozole alone in patients with advanced disease and no prior ET failed to demonstrate such a correlation [5,19], suggesting that the tumor level of CCNE1 mRNA might be a potential biomarker in ER+/HER2− ABC patients previously treated with ET. This evidence concerns the gene CDK4 and neoplasm.