Escalating doses of PD 0332991 in combination with bortezomib will be studied sequentially, with at least 3 patients in each dose level until the MTD is determined. PD 0332991 will be given orally, starting on Day 1 of each cycle, once a day for 12 days followed by 9 days without treatment. Bortezomib will be given by intravenous (IV) bolus or subcutaneously (taking 3 to 5 seconds to administer) on days 8, 11, 15 and 18. For dose levels 3a and 4b, bortezomib will be administered only on days 15 and 18. One cycle is defined as 3 weeks (21 days). Subjects will remain on treatment until they meet criteria for withdrawal from treatment described in section 5.4 (e.g., DLT, disease progression, unacceptable toxicity, or withdrawal of consent).