Pax-5 is a B-cell-specific activator protein (BSAP). In the early stages of B cell development, Pax-5 influences the expression of several B-cell-specific genes, such as CD19 and CD20. Pax-5 is expressed primarily in pro-, pre-, and mature B cells, but not in plasma cells. Pax5 is expressed in most B-cell malignancies (pre-B and mature B cell lymphomas/ leukemias). Over 90% of cases are positive for Pax5 in Hodgkin’s lymphoma, lymphoplasmacytic lymphoma, Merkel cell and small cell carcinoma. T-cell lymphomas, myeloma/ plasmacytoma and carcinoid tumors are negative for Pax5