VSIG1 (V-set and Ig domain-containing protein 1; also Glycoprotein A34) is a variably glycosylated 55 - 70 kDa member of the JAM family of proteins. It has restricted expression, being limited to testicular germ cells plus pancreatic duct and gastric epithelium. VSIG1 is likely to serve as an adhesion molecule. Mature human VSIG1 is 366 amino acids (aa) in length. It is a type I transmembrane glycoprotein that contains a 211 aa extracellular domain (ECD). The ECD contains one V-type (aa 22 - 132) and one C2-type Ig-like domain (aa 140 - 227). Over aa 22 - 234, human VSIG1 is 83% aa identical to both mouse and canine VSIG1. At least one potential splice variant exists in human. VSIG1 shows an insertion of 36 aa after Ser72 and a deletion of aa 133 - 387.