MECA-32 is a 50-55 kD homodimer glycoprotein primarily expressed on most endothelial cells in the embryonic and the adult mouse tissues, except in the brain and in skeletal and cardiac muscle, where it has more restricted distribution. In skeletal and cardiac muscle, the MECA-32 antigen is confined in small arterioles and venules. But in some conditions, such as inflammation or cardiac transplantation, MECA-32 expression can be induced on some negative-cardiac endothelial cells. In the brain, its expression occurs only in the circumventricular organs and the neurohemal tissues. During embryonic development, MECA-32 antigen is expressed on the brain vasculature up to day 16 of gestation, and disappears at day 17 and after.