PAX5 is a B-cell lineage specific activator protein (BSAP) that is essential for maintaining the identity and function of mature B cells during late B lymphopoiesis. It also plays a role in neural development and spermatogenesis. PAX5 is expressed in pro-, pre-, and mature B cells, and it is expressed in the vast majority of B-cell malignancies. Anti-PAX5 is a specific marker for the B cell lineage. A study of PAX5 immunohistochemistry by Jensen et al. suggested that PAX5 may be the only detectable marker of B lineage cells in lymphomas that lack or show equivocal CD45RB and CD20 expression. PAX5 is thus useful for a panel of antibodies for the identification of cellular origin of undifferentiated tumors. The expression of PAX5 in endocrine tumors has been shown to be high in Merkel cell carcinoma and small cell carcinoma, but not carcinoid tumor. PAX5 is also a marker for neuronendocrine carcinomas.