CD3E encodes the epsilon subunit of the T-cell receptor (TCR)-CD3 complex, a critical component of adaptive immunity located on T-lymphocyte surfaces 1. As part of the TCR-CD3 signaling apparatus, CD3E works with CD3D, CD3G, and CD247/CD3Z to transmit antigen-induced signals across the cell membrane when antigen-presenting cells activate the TCR 12. CD3E contains immunoreceptor tyrosine-based activation motifs (ITAMs) in its cytoplasmic domain that become phosphorylated by Src family kinases upon TCR engagement, recruiting and activating downstream signaling proteins like ZAP70 3. Notably, CD3E functions as a self-restrained signaling tuner: while some ITAM phosphorylation events activate T cells, mono-phosphorylated CD3E ITAMs selectively recruit inhibitory Csk kinase to attenuate TCR signaling 3. CD3E also serves as a receptor for ITPRIPL1, whose binding inhibits T-cell activation by blocking CD3E-ZAP70 interaction and downstream signaling cascades, enabling tumor immune evasion 4. Additionally, CD3E mediates TCR internalization and regulates cell surface expression through endocytosis. Mutations in CD3E cause Immunodeficiency 18, manifesting as severe combined immunodeficiency with complete T-cell developmental block 5.