A33 is a 45-kDa type I transmembrane glycoprotein that belongs to the CTX family of proteins. It is expressed on cells derived from primitive gut endoderm. These include normal large and small bowel epithelium, bladder epithelium, and virtually all colorectal carcinoma cells. Human and rodent A33 are approximately 300 amino acids in size and contain one V-type and one C-type Ig-like domain in their extracellular regions. A33 may function in epithelium-associated antigen presentation.