Exostosin-1 is an endoplasmic reticulum-resident type II transmembrane glycosyltransferase involved in the chain elongation step of heparan sulfate biosynthesis. Mutations in this gene cause the type I form of multiple exostoses.By screening a human chondrocyte cDNA library with cosmids that spanned breakpoints on chromosome 8q identified in patients with multiple exostoses type I, Ahn et al. (1995) identified a cDNA encoding a putative 746-amino acid protein with a molecular mass of 86.3 kD. Northern blot analysis detected expression of a 3.4-kb transcript in all tissues tested, with highest levels in liver. The authors noted that the breakpoint region in the EXT1 gene contains 2 identical polypyrimidine tracts (CCCCCCT) that are known to be deletion hotspots, similar to the retinoblastoma gene.