CD8 is a transmembrane glycoprotein composed of two disulfide-linked chains. It can be present as a homodimer of CD8α or as a heterodimer of CD8α and CD8β (PMID: 3264320; 8253791). CD8 is found on most thymocytes. The majority of class I-restricted T cells express mostly the CD8αβ heterodimer while CD8αα homodimers alone have been found on some gut intraepithelial T cells, on some T cell receptor (TCR) γδ T cells and on NK cells (PMID: 2111591; 1831127; 8420975). CD8 acts as a co-receptor that binds to MHC class-I and participates in cytotoxic T cell activation (PMID: 8499079). During T cell development, CD8 is required for positive selection of CD4-/CD8+ T cells (PMID: 1968084).