CCND3 (Cyclin D3) is a regulatory component of the cyclin D3-CDK4 (DC) complex that phosphorylates and inhibits members of the retinoblastoma (RB) protein family including RB1 and regulates the cell-cycle during G1/S transition. It hypophosphorylates RB1 in the early G1 phase. Cyclin D-CDK4 complexes are major integrators of various mitogenenic and antimitogenic signals. In immunohistochemistry, CCND3 has nuclear positivity in the cerebral cortex and cerebellum in the brain, adrenal gland, nasopharynx, colon and duodenum, pancreas, bladder, reproductive tissues, smooth muscle, and immune tissues. References: The UniProt Consortium. Nucleic Acids Res. 47: D506-515 (2019); Nucleic Acids Res. 2016 Jan 4;44(D1):D733-45, PMID:26553804;