Most PTCL, NOS show TCR gene rearrangements. Clonality is detected in more than 90% of cases when both TCRβ and TCRγ are tested. Immunoglobulin heavy-chain gene rearrangements are noted in about a third of cases, usually associated with the presence of EBV+ B-cells or evidence of B-cell expansion. With cytogenetics, karyotypic abnormalities are detected in virtually all cases. Complex karyotypes are seen with large cell morphology and are linked to poor outcomes. Trisomy 3 is frequently seen in the lymphoepithelioid variant. The rare follicular variant of PTCL, NOS shows recurrent translocation t(5;9)(q33;q22). Based on the gene expression studies, PTCL, NOS are found to be a heterogenous group with different subgroups having different molecular profiles. These profiles may have prognostic relevance.