CD7, also known as GP40 or T-cell antigen CD7, is a 40 kDa transmembrane glycoprotein that belongs to the immunoglobulin superfamily. It is encoded by the CD7 gene located on chromosome 17 and is expressed on the surface of T cells, natural killer (NK) cells, and their precursors. CD7 plays a crucial role in immune regulation by participating in T-cell activation, providing co-stimulatory signals, and facilitating cell adhesion. It is one of the earliest antigens to appear on T-lymphocyte lineage cells and is highly expressed in various stages of T, B, and myeloid cell differentiation. Additionally, CD7 is involved in T-cell infiltration in tumors, making it an important target for cancer immunotherapy.