The protein of CCND1 (Cyclin D 1) belongs to the highly conserved cyclin family, functioning as regulators of CDK kinases. This cyclin forms a complex with and functions as a regulatory subunit of CDK4 or CDK6, whose activity is required for cell cycle G1/S transition. This protein has been shown to interact with tumor suppressor protein Rb and the expression of this gene is regulated positively by Rb. Amplification of this gene, which alters cell cycle progression, are observed frequently in a variety of tumors. Cyclin D1 and the mechanisms it regulates have the potential to be a therapeutic target for cancer drugs, including inhibition of Cyclin D1, induction of Cyclin D1 degradation, and inhibition of Cyclin D1/CDK 4/6 complex.