V-set and immunoglobulin domain-containing protein 4 (VSIG4) is also known as Protein Z39Ig, is a type I transmembrane glycoprotein. VSIG4 is a B7 family-related protein and an Ig superfamily member. VSIG4 contains two Ig-like (immunoglobulin-like) domains. VSIG4 is abundantly expressed in several fetal tissues. In adult tissues, the highest expression of VSIG4 is in lung and placenta. VSIG4 functions as a negative regulator of T cell activation, and may be involved in the maintenance of peripheral T cell tolerance, and is also identified as a potent suppressor of established inflammation. VSIG4 is a phagocytic receptor, strong negative regulator of T-cell proliferation and IL2 production.