A protein that has a core domain composition consisting of an extracellular N-terminal domain that adopts an immunoglobulin fold, a transmembrane domain, and an intracellular C-terminal domain with a single copy of the Immunoreceptor tyrosine-based activation motif (Pfam:PF02189) (ITAM). It constitutes the invariant subunit of the T cell antigen receptor (TCR). TCR is a surface receptor on T cells responsible for recognizing MHC-restricted antigens and initiating the cellular immune response. [ http://www.ncbi.nlm.nih.gov/pubmed/16473826 http://www.ncbi.nlm.nih.gov/pubmed/1724736 ]