The COL18A1 gene encodes the alpha chain of type XVIII collagen. There are three isoforms of the alpha 1 subunit of collagen XVIII, which form three different lengths of the collagen XVIII protein. The short isoform of collagen XVIII is found in basement membranes throughout the body, including several parts of the eye. The two longer isoforms are found primarily in the liver. The function of collagen XVIII is not well known, but it appears that all of the isoforms are involved in determining the retinal structure as well as in the proper closure of the neural tube. Endostatin, which is produced by proteolytic processing at several endogenous cleavage sites in the C-terminal domain, inhibits endothelial cell proliferation and angiogenesis. Defects in this protein are the cause of Knobloch syndrome, an autosomal recessive disorder characterized by vitreoretinal and macular degeneration and occipital encephalocele.