<qa>  What is the primary endpoint of the study?This study is a phase I, single arm, open-label trial of PD0332991 in combination with paclitaxel in patients with Rb-expressing metastatic breast cancer. Patients will be treated as shown in the schema below. Up to 20 patients are anticipated to be enrolled to reach the MTD of PD0332991 in combination with Paclitaxel. Once the MTD is established, an additional expanded cohort of 10 patients will be enrolled at that dose to estalish the RP2D, obtain additional safety data and perform exploratory biomarker studies. The primary endpoint will be assessed after one cycle of therapy. Patients will remain on study until dose limiting toxicity, disease progression or physicianpatient discretion. Safety assessment will continue for the durationof patient participation. 