PAX-5 encodes for B-cell-specific activator protein (BSAP), a marker for B-cells, including B-lymphoblastic neoplasms and maturation stage. It is found in most cases of mature and precursor B-cell non-Hodgkin lymphomas/leukemias. In approximately 97% of cases of classic 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 do express PAX-5, save for those with terminal B-cell differentiation. T-cell neoplasms do not stain with anti-PAX-5. There is a strong association with CD20 expression.^1-4