Endomucin is a type I membrane protein of 248aa (75kD) and shows no signicant homology to any known glycoprotein. As a typical mucin-like glycoprotein, endomucin has a high content of serine and threonine residues, suggesting strong O- glycosylation; the sensitivity to O-sialoglycoprotein endopeptidase indicates that endomucin is also a sialomucin. Endomucin is an endothelial-specific sialomucin. It is a constitutively expressed endothelial cell surface protein that is found on all venules but is absent from high endothelial venule cells (HEV) of peripheral and mesenteric lymph nodes as well as Peyer's patches, the specialized site for most efficient lymphocyte trafficking. This could indicate an anti-adhesive function of endomucin, as demonstrated for other sialomucins. Mucosal addressin cell adhesion molecule 1 (MAdCAM-1) is another cell adhesion molecule that contains a mucin-like domain and is expressed on HEV in Peyer's patches, mesenteric lymph nodes and on venules in intestinal lamina propria. In the HEV of mesenteric lymph nodes, the mucin-like domain of a subpopulation of MAdCAM-1 molecules contains sulfated carbohydrate side chains that interact with L-selectin. The presence of three putative protein kinase C phosphorylation sites in the cytoplasmic tail of endomucin indicates that endomucin has the capacity to be a signaling molecule.