Cyclin D1 (CCND1) is a gene that encodes a protein that functions in the regulation of CDK kinases in the cell cycle. Cyclin D1 (CCND1) regulates G1 to S-phase cell cycle transition alongside its binding partners, CDK4 and CDK6, via phosphorylation and inactivation of the retinoblastoma protein (pRb).

Fusions, rearrangements, missense mutations, nonsense mutations, silent mutations, and in-frame deletions and insertions are observed in cancers such as endometrial cancer, intestinal cancer, and stomach cancer. First identified in 1991, CCND1 has been found amplified in about 9-30% of breast cancer cases. CCND1 is altered in 3.80% of all cancers with breast invasive ductal carcinoma, invasive breast carcinoma, bladder urothelial carcinoma, breast invasive lobular carcinoma, and lung adenocarcinoma having the greatest prevalence of alterations.