Control of the cell cycle is regulated by a multitude of cellular events and processes. The cyclin dependent kinases (CDK) regulate many of these pathways and constitute an active complex when associated with their cyclin partners. This activity is controlled primarily by phosphorylation, which determines subcellular localization of the CDK/cyclin complex. Some phosphorylation events control the function of cytoplasmic retention sequences while other events regulate nuclear localization and export sequence function. Cyclin and cyclin-dependent kinase inhibitor (CKI) levels are regulated by ubiquitination and degradation via the ubiquitin proteasome pathway. A variety of CKI proteins associate with these complexes and modulate access to regulatory domains on cyclins. Additional complexity is generated as the controlled protein levels of each cyclin oscillate with the stages of cell cycle. Increased expression of Cyclin D1 is associated with certain types of cancer and may associate with