Cyclin D1, also known as Bcl-1, CCND1, or PRAD1, is a 36 kDa nuclear protein that plays a key regulatory role during the G1 phase of the cell cycle by regulating the activity of Cdk proteins. Cyclin D1 forms a complex with Cdk4, then binds to and phosphorylates Rb protein, triggering cells to progress from G0/G1 to S and thus driving cellular proliferation. Cyclin D1 protein overexpression is found in the majority of human breast cancers and is usually linked with poor prognosis. Overexpression of cyclin D1 is a well-established criterion for the diagnosis of Mantle Cell Lymphoma, and also has been used to distinguish malignant breast carcinomas from premalignant breast lesions.