Capecitabine, the comparator in this trial, is an orally-administered fluoropyrimidine derivative and has shown high efficacy and low toxicity in metastatic breast cancer patients. Palbociclib is a CDK4/6 inhibitors, in combination with endocrine therapy showed marked advance in hormone receptor-positive MBC in the post-menopausal setting. After a median follow-up of 16.5 months, preliminary results from Part 1 of this Phase 2 trial suggest that the combination of PD-0332991 with letrozole is superior to letrozole alone, and improved objective response and disease control rates (52% vs 32% and 76% vs 47%, respectively) in patients treated with the combination. These remarkable results may contribute to have much benefit with endocrine therapy for premenopausal women. Most importantly, recent PALOMA-3 trial revealed superior results of adding palbociclib to fulvestrant (median PFS 9.2 vs 3.8 months, P<0.001).