CD3ε, also known as the epsilon chain of the CD3 complex, is a crucial component of the T cell receptor (TCR) complex, which plays a central role in T cell activation and antigen recognition. CD3ε is particularly important in the context of T cell-mediated immune responses. It has been identified as a key target in the development of immunotherapies for cancer. Recent research has also highlighted the role of CD3ε in CAR-T cell therapy. By incorporating the CD3ε cytoplasmic domain into a chimeric antigen receptor (CAR), the antitumor activity of CAR-T cells can be improved.