PAX-5 encodes the B-cell-specific activator protein (BSAP), a marker for B-cells, including B-lymphoblastic neoplasms and maturation stages. It is present in most cases of mature and precursor B-cell non-Hodgkin’s lymphomas/leukemias. In approximately 97% of cases of classical Hodgkin lymphoma, Reed Sternberg cells express PAX-5.4 PAX-5 is not detected in multiple myeloma and solitary plasmacytoma, making it useful for such differentiation.1,3 Diffuse large B cell lymphomas express PAX-5, with the exception of those with terminal B cell differentiation. T cell neoplasms do not stain with anti-PAX-5 and there is a strong correlation with CD20 expression.