The neoplastic T-cells in PTCL, NOS are positive for pan-T-cell related antigens such as CD3, CD2, CD5, and CD7 (shown here); however, one or more of these antigens are frequently lost. Most cases are CD4+ve/CD8-ve. A significant proportion have double negative (CD4-ve/CD8-ve) phenotype. Other phenotypes (CD4-ve/CD8+ve and double positive) are rare. CD4+ve cases have better prognosis. Double-negative immunophenotype is linked to adverse outcome. About 30% to 40% of PTCL, NOS, express markers of cytotoxic T-cells such as T-cell intracellular antigen-1 (TIA-1). This feature is associated with CD8+ve phenotype and is indicative of poor prognosis. CD10 expression is usually lacking in PTCL, NOS, except in the follicular variant. Expression of the activation marker CD30 may be seen in occasional tumor cells. Some cases may express it more diffusely. Coexpression of CD15 and CD30 (usually associated with classical Hodgkin’s lymphoma) may be seen in some PTCL, NOS. A small subset of PTCL, NOS, seen mainly in elderly males, express dim CD20 and pursue an aggressive clinical course. Image courtesy of: Jared Block, MD, Carolinas Pathology, Charlotte, NC; Sciplicity, LLC. Image courtesy of: Jared Block, MD, Carolinas Pathology, Charlotte, NC; Sciplicity, LLC.