Cyclin D1, is cell cycle regulator and it is over expressed in a wide variety of human neoplasms. Cyclin D1 forms a complex with regulatory subunit of CDK4 or CDK6 kinases and it is required for cell cycle G1/S transition. The expression is maximal in G1 and minimal in S phase of cell cycle. Cyclin D1 expression is located mainly to the proliferative zone of normal epithelial tissues. Localization of the cyclin D1 is mainly nuclear. Cyclin D is useful for lymphoma diagnostic, especially diagnosis of mantle cell lymphoma.