CD3d, also known as CD3 delta chain, is a protein-coding gene located on chromosome 11q23.3. The protein encoded by CD3D is a single-pass type I membrane protein and is a component of the T-cell receptor (TCR) complex, which is crucial for T-cell development and signaling. It forms part of the CD3 complex along with other subunits like CD3γ, CD3ε, and CD3ζ, and it interacts with either the TCRα/β or TCRγ/δ heterodimer to form the TCR/CD3 complex on the T-cell surface. The CD3d protein is also a target for immunotherapies. For example, bispecific antibodies that target CD3d along with tumor-associated antigens can redirect T cells to attack cancer cells, making it a promising target for cancer immunotherapy.