CDKN1B (cyclin dependent kinase inhibitor 1B) is a cell cycle inhibitor enzyme whose primary function is to suppress the cell-division cycle at G1. Mutations in CDKN1B are associated with hereditary prostate cancer. Furthermore, somatic inactivating mutations are common in luminal breast cancers and neuroendocrine tumors of the small intestine. In immunohistochemistry, CDKN1B has nuclear and cytoplasmic positivity in the adrenal gland, breast, central nervous system, cervix, large intestine, liver, lung, esophagus, ovary, prostate, stomach, thyroid, and urinary tract. References: Front. Endocrinol. 2018. 9:393, DOI: 10.3389/fendo.2018.00393; Cancer Res. 2004 Mar 15;64(6):1997-9, PMID: 15026335