The function of VTCN1 (V-set domain containing T cell activation inhibitor 1, ENSG00000134258) is as follows. Negatively regulates T-cell-mediated immune response by inhibiting T-cell activation, proliferation, cytokine production and development of cytotoxicity. When expressed on the cell surface of tumor macrophages, plays an important role, together with regulatory T- cells (Treg), in the suppression of tumor-associated antigen-specific T-cell immunity. Involved in promoting epithelial cell transformation. {ECO:0000250|UniProtKB:Q7TSP5, ECO:0000269|PubMed:15878339, ECO:0000269|PubMed:16606666, ECO:0000269|PubMed:17509674, ECO:0000269|PubMed:17875732}.