<qa>  What is the MTD of PD0332991?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. 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 establish the RP2D, obtain additional safety data and perform exploratory biomarker studies. 