APOBEC3B is a repair gene and cytidine deaminase involved in somatic hypermutation. The deregulation or dysfunction of APOBEC3B and other members of the APOBEC family has been implicated as a direct source of mutagenesis in breast and other cancers. APOBEC3B is upregulated in a number of breast cancers, and this upregulation correlates with a mutation spectrum (kataegis) in these tumors consistent with APOBEC function and may be a simultaneous or upstream consequence of ERBB2 amplification. APOBEC mutation signatures have also been identified in lung cancer, as well as evidence of APOBEC3B upregulation in lung cancer and a correlation between increased expression and worse prognosis in non-small-cell lung carcinoma (NSCLC), making it an important potential biomarker for the disease. In immunohistochemistry, APOBEC3B has low levels of nuclear positivity in tissues throughout the body, with highest expression in peripheral blood leukocytes and in the spleen, heart, thymus, prostate, ovary, and testes. References: Genome Biol. 2016; 17: 185, PMID: 27634334; eLife. 2013; 2: e00534, PMID: 23599896; Oncogene. 2018 Jul;37(29):3924-3936, PMID: 29695832; Elife. 2013 Apr 16;2:e00534. doi: 10.7554/eLife.00534, PMID: 23599896; Br J Cancer. 2017 Jun 27; 117(1): 113–123, PMID: 28535155;