CCND1 is a member of the cyclin family with nuclear expression in the majority of adult tissues. CCND1 is involved in regulating the G1/S transition via complex formation with CDK4 and CDK6 and inhibition of RB1. CCND1 is also involved in regulating proliferation, differentiation and growth by binding to receptors such as estrogen receptor alpha (ESR1), androgen receptor (AR), and thyroid hormone receptors. Because of its regulatory functions in cell cycle progression, CCND1 is a frequent target of amplification or mutation as part of tumorigenesis. In immunohistochemistry, CCND1 has positive nuclear staining in squamous epithelium of the tonsil and also in mantle cell lymphoma. In cancer, it stains positively in multiple myelomas, some parathyroid adenomas and parathyroid carcinomas, and it is overexpressed in a number of cancers including breast cancer. References: Molecular Medicine Reports 9.1 (2014): 137-143, DOI: 10.3892/mmr.2013.1774; Cell. 1997 Feb. 88 (3): 405–15, PMID: 9039267; Molecular Endocrinology. 2001 May. 15 (5): 797–811, PMID: 11328859