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Reviewer: Nat Pernick, M.D. (see Reviewers page)
Revised: 21 June 2013, last major update January 2013
Copyright: (c) 2002-2013, PathologyOutlines.com, Inc.
PubMed Search: Pax-5 [title]
Broken links/comments: click here or email CopyrightPathOut@gmail.com, subject: StainsPAX5


● Also called BSAP, a B cell-lineage specific activator protein at 9q13
● Member of paired box (PAX) family of transcription factors, which have a novel, highly conserved DNA-binding motif, known as the paired box; encodes BSAP expressed at early stages of B-cell differentiation, also in developing CNS and testis (Wikipedia)
● Detected in B cells from pro-B cell stage to plasma cell stage where it is downregulated (Clin Med Res 2010;8:84)
● Required for progression of B cell development beyond the early pro-B cell stage (Cancer Res 2011;71:7345)

Uses by pathologists

● Detection of pre-B cells (PAX5+, more sensitive and specific than CD20)
● Diagnosis of Reed-Sternberg cells in classic Hodgkin lymphoma (PAX5+) versus T / null cell anaplastic large cell lymphoma (PAX5-, although rarely positive in T cell lymphomas, Mod Pathol 2010;23:593)
● Diagnosis of lymphoplasmacytic lymphoma / plasmacytoid differentiation in marginal zone lymphoma (PAX5+) versus plasmacytoma (PAX5-, Am J Surg Pathol 2002;26:1343)

Micro images

Various images

Rare PAX5+ anaplastic large cell lymphoma

Uterus (endometrium): mesonephric adenocarcinoma

Positive staining - normal

● B cells from pro-B cell stage to plasma cell stage, where it is downregulated

Positive staining - disease

● Most B cell malignancies (pre-B and mature B cell lymphomas / leukemias)
● B cell lymphomas following rituximab therapy
● Hodgkin lymphoma (Reed-Sternberg cells, 97%)
● Lymphoplasmacytic lymphoma
● Merkel cell and small cell carcinoma (93% and 73%, Am J Surg Pathol 2005;29:687); high grade neuroendocrine carcinomas (Am J Clin Pathol 2008;129:556, Am J Clin Pathol. 2006;126:798)

Negative staining

● Carcinoid tumor, embryonal carcinoma, myeloma, plasmacytoma, seminoma, T cell lymphoma

Molecular images

Rare PAX5 anaplastic large cell lymphoma shows extra copies of the PAX5 gene locus

End of Stains > PAX5

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