 The CD3γ, CD3δ, and CD3ε chains are highly related cell-surface proteins of the immunoglobulin superfamily containing a single extracellular immunoglobulin domain.A structure of the extracellular and transmembrane regions of the CD3γε/CD3δε/CD3ζζ/TCRαβ complex was solved with CryoEM, showing for the first time how the CD3 transmembrane regions enclose the TCR transmembrane regions in an open barrel.Containing aspartate residues, the transmembrane region of the CD3 chains is negatively charged, a characteristic that allows these chains to associate with the positively charged TCR chains.