CD6 is a type I membrane protein expressed by thymocytes, mature T cells, a subset of B cells known as B1 cells, and some cells in the brain. CD6 is a member of the scavenger receptor cysteine rich (SRCR) family of proteins, which includes the leukocyte antigen CD5. The extracellular domain of CD6 is composed of three SRCR domains and has the same domain organization as CD5. The ligand for CD6 is CD166 also known as activated leukocyte cell adhesion molecule. Through its interaction with CD166 in the thymus, CD6 may have a role in T cell development.