VSIG4 (V-set and immunoglobulin domain containing 4), also known as complement receptor of the immunoglobulin superfamily (CRIg) and Z39Ig, is a type I transmembrane glycoprotein. It is a B7 family-related protein and an Ig superfamily member. In contrast to the B7 family members which contain two IgG domains, VSIG4 contains one complete V-type Ig domain and a truncated C-type Ig domain. VSIG4 is exclusively expressed on tissue resident macrophages and binds to multimers of C3b and iC3b that are covalently attached to particle surfaces. No VSIG4 expression appears to be present in T and B cells. 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. Mouse VSIG4 is synthesized as a 28 amino acid precursor that contains a signal sequence, an V-type Ig domain (aa 36-115), one potential N-linked glycosylation site, and a single transmembrane domain. The V-type Ig domain of mouse VSIG4 shares 86% and 8% aa sequence identity with the V-type Ig domains of rat and human VSIG4, respectively.