BSAP, also known as paired box protein 5 (Pax5), is a transcription factor expressed in B cells. BSAP is a member of the PAX gene family that encodes transcription factors involved in B-cell development. BSAP expressed in pro-, pre-, and B cells, but not in plasma cells. Targeted disruption of the BSAP gene in mice blocks B-cell development at the pro-B0cell stage, suggesting a role for BSAP in the control of B-cell development. During embryogenesis, BSAP expression is transiently expressed in the developing central nervous system. Later, BSAP expression is detected in the fetal liver where it correlates with the onset of B lymphopoiesis. Thus, BSAP may not only play an important role in B-cell development but also in neural development. In immunohistochemistry, it can be difficult to distinguish classic Hodgkin lymphoma (CHL) from anaplastic large cell lymphoma if T- and null-cell type. In CHL, Hodgkin and Reed-Sternberg cells (HRS cells) express BSAP, whereas HRS cells mostly do not display B-cell antigens such as CD19 or CD20. BSAP is a useful marker for the diagnosis of B-cell non-Hodgkin lymphoma and Hodgkin lymphoma and neuroendocrine tumors despite its expression in subset of epithelial malignancies.