CD81 (Mr 25kD) is a member of the transmembrane 4 integral membrane protein family CD81/TAPA-1 is an integral membrane protein expressed on a variety of cell types, and has a high degree of sequence homology between human and mouse.1 CD81 is expressed on thymic stromal cells where it plays an important role in the transition of gamma/delta+ T cells to more mature T cells with alpha/beta T cell receptors.2 Immunohistochemical staining has revealed that its expression is localized to the subcapsular region of the thymus and, specifically, on cells that have distinct clustering patterns. It has been speculated that the ligand for CD81 is the pre-T cell receptor, which is composed of a TCR beta chain and glycoprotein pTalpha.2 This antibody can block thymocyte interaction with CD81 in vitro.2