At the plenary session of the AACR, the phase II PALOMA-1 trial results were presented in which the cyclin-dependent kinase 4/6 inhibitor, palbociclib, was combined with letrozole and compared with letrozole alone in the front-line setting for the treatment of women with metastatic disease. The trial consisted of two parts: in part one, 66 post-menopausal women with ER-positive, HER2-negative breast cancer were enrolled and in part two, 99 patients were additionally screened for CCND1 amplification, the loss of p16, or both. There was a significant improvement in median progression-free survival for women treated with the palbociclib combination (20.2 months) compared with letrozole alone (10.2 months). These treatment effects were retained when both parts of the trial were assessed separately.