Background: The G1/S checkpoint of the cell cycle is frequently dysregulated in breast cancer (BC). Initial efficacy of PD0332991, a potent oral inhibitor of cyclin-dependent kinases (CDKs) 4/6 was shown in a variety of solid tumors and in combination with letrozole in a randomized phase II trial. Methods: We performed a phase II, single arm trial of PD0332991 in women with advanced BC. The primary objectives were safety and efficacy. Eligible patients had histologically-confirmed, stage IV BC with primary or metastatic tumor positive for retinoblastoma (Rb) protein expression, measureable disease by RECIST and adequate organ function/performance status. PD0332991 was given at 125 mg orally, days 1 – 21 of a 28-day cycle. Tumor was assessed every 2 cycles. A two-stage statistical design was employed. Secondary objectives included predictive biomarker assessment. Results: 36 patients were enrolled; 28 who completed cycle 1 are reported: 18 (64%) HR+/Her2-, 2 (7%) HR+/Her2+ and 8 (29%) HR-/Her2-. 90% had prior chemotherapy for metastatic disease (median 3 lines); 78% had prior hormonal therapy (median 2 lines). Grade 3/4 toxicities were limited to transient neutropenia (50%) and thrombocytopenia (21%). One episode of neutropenic sepsis occurred in cycle 1 in patient with 6 prior chemo regimens. All other toxicities were grade 1/2. Treatment was interrupted in 7 (25%) and dose reduced in 13 (46%) pts for cytopenias. For response data see table. Responses occurred at dose levels as low as 50 mg. Median PFS (months, 95% CI) was 4.1 (2.3,7.7) for ER+/Her2-, 18.8 (5.1,∞) for ER+/Her+ and 1.8 (0.9,∞) for ER-/Her2-. 27/28 patients discontinued study for progressive disease (PD); 1 due to patient preference. Conclusions: Therapy with PD0332991 alone is well-tolerated and demonstrates response or prolonged stable disease (SD) in patients with BC despite prior hormonal and chemotherapy. Expansion within subtypes and molecular predictors of response are being investigated. Clinical trial information: NCT01037790.