CD5, also known as Leu-1, is a type I transmembrane protein belonging to the scavenger receptor cysteine-rich (SRCR) superfamily. It is expressed on thymocytes, T cells and the B1a subpopulation of B cells. CD5 binds the C-type lectin, CD72, and modulates signaling through the antigen receptors on T and B cells. CD5 has been shown to provide either positive or negative co-stimulatory signals depending on cell type to regulate immune responses.