During each cell cycle cyclins undergo periodic accumulation and destruction. As key regulators of the cell cycle the cyclins control important transitions by acting as regulatory subunits of the Cdks. Early in the G1 phase of the cell cycle, cyclin D1 induction is followed by cyclin E induction. This sequential progression is marked early on in G1 by the activation of Cdk4 and in mid to late G1 by the activation of Cdk2 and the hyperphosphorylation of pRB. The final transition into S phase is thought to be dependent on the increased expression and association of cyclin E and Cdk2. In a recent study, Cyclin D1 regulates cellular metabolism, fat cell differentiation and cellular migration. Cyclin D1 is also involved in development and cancer. Cyclin D1 has also been linked to the development and progression of several cancers including breast, bladder, esophagus, and lung.