CD6 (Cluster of Differentiation 6) is a cell surface glycoprotein that is involved in T cell activation and a member of the scavenger receptor cysteine rich protein superfamily (SRCRSF). CD6 is expressed at low levels on immature thymocytes and at high levels on mature thymocytes. The majority of peripheral blood T cells, a subset of B cells, and a subset of neuronal cells express CD6. CD6 appears to play a role as both a costimulatory molecule in T cell activation and as an adhesion receptor. Studies demonstrating a mitogenic effect for T cells with some CD6 specific monoclonal antibodies, in conjunction with either accessory cells or PMA and anti-CD2 mAb, support the concept of CD6 as a co stimulatory molecule.