The alpha-beta heterodimeric T-cell antigen receptor binds antigen in association with major histocompatibility complex proteins on host cell surfaces. These 2 disulfide-linked glycoproteins, TCRA and TCRB are associated on T-cell surfaces with a complex of proteins called CD3 (formerly T3). Human CD3 consists of at least 4 proteins: gamma, delta , epsilon, and zeta . The CD3-gamma/CD3-delta gene pair is within 300 kb of the CD3-epsilon gene and therefore these genes form a tightly linked cluster on 11q23. The clustering may be significant in terms of their simultaneous activations during T-cell development.