In immunology, the CD3 (cluster of differentiation 3) T-cell co-receptor is a protein complex and is composed of four distinct chains. In mammals, the complex contains a CD3gamma chain, a CD3delta chain, and two CD3epsilon chains. This complex plays an important role in coupling antigen recognition to several intracellular signal-transduction pathways. CD3 zeta, also known as T-cell receptor T3 zeta chain or CD247, is a 164 amino acids protein with a predicted weight of 18.696 Kda. It plays an important role in coupling antigen recognition to several intracellular signal-transduction pathways.