CD6 is a member of the group B scavenger receptor cysteinerich(SRCR) superfamily. CD6 is a type I membrane glycoprotein and contains three extracellularSRCR domains. CD6 is expressed at low levels on immature thymocytes and at high levels on mature thymocytes. The majority of peripheral blood T cells, a subsetof B cells, and a subset of neuronal cells express CD6. Mouse CD6 is a 626 amino acid (aa) protein with a 24 aa sequence, a 372 aa extracellular domain, and a204 aa cytoplasmic region. The role of CD6 has not been fully elucidated. However, it appears to play a role as both a costimulatorymolecule in T cell activation and as an adhesion receptor.CD6/ALCAM interactions have been postulated to play a role in thymocyte development. The CD6 intracellular domain contains regions that can interact with SH2 or SH3 containing proteins. However,the signaling pathways have not been elucidated.