Cyclin D1 (PRAD-1, bcl-1) is one of the key cell cycle regulators, and functions in association with cdk4 and/or cdk6 by phosphorylating the Rb protein. Cyclin D1 is a putative proto-oncogene overexpressed in a wide variety of human neoplasms including mantle cell lymphomas (MCL). In addition, cyclin D1 positively regulates protein phosphorylation, mammary gland epithelial cell proliferation, and fat cell differentiation. In humans, the CCND1 gene encoding cyclin D1 is present on chromosome 11. Cyclin D1 is a cytoplasmic and nuclear protein that is synthesized during G1 phase and assembles with either cyclin-dependent kinase 4 (CDK4) or CDK6 in response to growth factor stimulation. Cyclin D1 staining is a useful tool for investigating cell cycle biology and related cancers. Anti-Cyclin D1 is used for differentiating mantle cell lymphomas (positive stain) from CLL/SLL and follicular lymphomas (negative stain). Hairy cell leukemia and plasma cell myeloma also react lightly to Anti-Cyclin D1.