NK/T-cell neoplasms are almost exclusively extranodal and include extranodal NK/T-cell lymphoma (nasal type), aggressive NK-cell leukemia, chronic lymphoproliferative disorder of NK cells, and NK lymphoblastic leukemia/lymphoma (WHO Classification, 2008). CLINICAL: Extranodal NK/T-cell lymphoma (ENKTCL) is more common in Asia and South America and quite rare in US and Europe. It affects mainly adults (median age 53 years) with a slight male predominance. It usually presents as an ulcerative, necrotizing mass in the nasal cavity (most common location), other parts of upper aerodigestive tract, skin, gastrointestinal tract, testis, or soft tissues. It is a highly aggressive tumor with poor prognosis despite chemotherapy. There is strong association with EBV infection. MORPHOLOGY: The tumor shows angiocentric and angioinvasive growth leading to ischemic necrosis and ulceration of the involved tissues. The tumor is composed of small, medium-sized, or large cells which are admixed with apoptotic bodies and inflammatory cells. IMMUNOPHENOTYPE: A typical case of NK/T-cell lymphoma has the following profile: POSITIVE for CD2, cytoplasmic CD3å, CD56 (rare cases are CD56 negative), cytotoxic molecules (perforin, granzyme B, TIA-1), CD43, CD45RO, Fas (CD95) and Fas-ligand (CD178), CD25 (15% cases), CD30 (50% cases; weak and focal). NEGATIVE for surface CD3, CD16, CD57, T-cell associated markers (CD4, CD5, and CD8), CD20 (rare cases may be positive). GENETICS: The T-cell receptor (TCR) and immunoglobulin genes are not rearranged (i.e. germline in configuration) in the majority of cases. Association with EBV is seen in 90% to 100% of cases and can be detected by in-situ hybridization for Epstein-Barr virus-encoded RNA (EBER). Mutations in p53 gene have been detected in 25% to 60% of cases. They correlate with large cell morphology and advanced clinical stage.