CD3D is one of the components of the T-cell receptor (TCR)/CD3 complex and functions in the signal transduction of T-cell activation. A TCR-mediated signal is conducted across the plasmalemma through the CD3 chain upon extracellular binding of the TCR to antigen-presenting cells. All CD3 chains, which include CD3G, CD3D, CD3E, and CD3Z, possess immunoreceptor tyrosine-based stimulation motifs in their cytoplasmic structural domains. These motifs undergo phosphorylation when acted upon by the Src family protein tyrosine kinases LCK and FYN, resulting in the activation of downstream signaling pathways. In addition, CD3D functions in thymocyte differentiation. Thymocytes fail to differentiate appropriately without a functioning TCR/CD3 complex. Moreover, previous studies reported that CD3D could act as a biomarker for cancers.