PAX5 is a paired box family transcription factor involved in B-cell differentiation and in the development of the testis and central nervous system. In normal tissue, PAX5 has specific nuclear expression in pre-B cells and is downregulated in plasma cells. In cancer it is positive in pre B-cell acute lymphoblastic leukemia and classical Hodgkin leukemia (Reed-Sternberg cells), merkel cell carcinomas, small cell carcinomas and high grade neuroendocrine carcinomas. PAX5 is a useful marker for distinguishing lymphoplasmacytic lymphomas (PAX5 positive) from plasmocytomas (negative). References: Adv Anat Pathol. 2007 Sep;14(5):323-34, PMID: 17717432; Am J Surg Pathol. 2005;29:687, PMID: 15832095; Am J Clin Pathol 2008;129:556, PMID: 18343782, Am J Clin Pathol. 2006;126:798, PMID: 17050077; Am J Surg Pathol 2002;26:1343, PMID: 12360049