CD1a is a non-polymorphic cell surface glycoprotein (45 to 55 kDa) related to the major histocompatibility complex class I and is expressed in association with β-microglobulin. CD1a is predominantly expressed in dendritic cells and thymic cortical cells and is positive in 70% of thymocytes, but is often not expressed in early thymic cells or mature cells. In normal tissue, anti-CD1a reacts with cortical thymocytes, Langerhans cells, interdigitating cells and rare antigen-presenting cells of the lymph node. CD1a positivity has also been found in Langerhans cell histiocytosis (histiocytosis X)3 and a subset of pre-T-lymphoblastic lymphomas/leukemias (cortical T-LBL/L). CD1a will be used for the differentiation of thymic tumors from primary lung tumors.