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A transcription factor expressed throughout B-cell maturation, is detected in most B-cell neoplasms including those that lack expression of mature B-cell markers, such as classical Hodgkin lymphoma (cHL), non-Hodgkin lymphoma, B-lymphoblastic leukemia, and B-cell lymphomas following rituximab therapy. This biopsy shows diffuse positive PAX5 in the neoplastic cells regimen (arrow) supporting a B-cell derivation. The strong expression is also unusual for classical Hodgkin Lymphoma and hence the gray zone designation is preferred here.