Preclinical results in breast cancer triggered the beginning of the randomized Phase II trial (PALOMA-1) involved 165 advanced-stage HR+ and HER2- postmenopausal breast cancer patients who had not received prior systemic treatment.637 The study demonstrated that combining palbociclib with the aromatase inhibitor letrozole significantly improved median progression-free survival (PFS) to 20.2 months, compared to 10.2 months with letrozole alone. This evidence concerns the gene ERBB2 and breast cancer.