CD1a is a non-polymorphic, major histocompatibility complex, class I-related cell surface glycoprotein (45 to 55 kDa) and is expressed in association with β-microglobulin.¹ CD1a is predominantly expressed in dendritic cells and thymic cortical cells, and is positive in 70% of thymocytes, and is often not expressed in early thymic cells or mature cells. In normal tissues, anti-CD1a reacts with cortical thymocytes, Langerhans cells, interdigitating cells, and rare antigen-presenting cells of the lymph node.² CD1a positivity has also been seen in Langerhans cell histiocytosis (histiocytosis X)3, and a subset of pre-T lymphoblastic lymphoma/leukemia (cortical T LBL/L).^4,5 CD1a is used for the classification of thymoma, and cortical thymoma; CD1a can be used for the differentiation of thymic tumors from primary lung tumors.