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).