The mature VAP-1 molecule is a 170 kDa homodimeric glycoprotein that consists of two 90 kDa subunits held together by disulfide bonds. VAP-1 has a large extracellular domain, a single-pass transmembrane domain, and a short cytoplasmic tail. The molecule has abundant sialic acid decorations that are essential to its adhesive function, because VAP-1 is unable to mediate lymphocyte adhesion to desialylated vessels. The leukocyte ligand for VAP-1 is currently unknown. Induction of VAP-1 has been shown at sites of inflam-mation, such as in inflammatory bowel diseases and chronic dermatoses, where expression of VAP-1 is clearly increased. It is constitutively expressed on hepatic endothelium playing a critical role in regulation of T-cell recirculation to the liver. Strong expression of VAP-1 on tumor endothelium distinguishes human hepatocellular carcinomas from colorectal hepatic metastases.