PAX-5 is a member of the paired box (PAX) family of transcription factors, which are key regulators in early development. The PAX-5 gene encodes the B-cell lineage specific activator protein (BSAP), whose expression is limited to early stages of B-cell differentiation. Anti-PAX-5 is useful in differentiating between classic Hodgkins lymphoma versus multiple myeloma and solitary plasmacytoma, as the protein is expressed in mature and precursor B-cell non-Hodgkins lymphomas/leukemias while being absent from the other two conditions. Diffuse large B-cell lymphomas are positive for PAX-5, with the exception of those with terminal B-cell differentiation, and T-cell neoplasms do not stain with Anti-PAX-5.

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