Glycosyltransferases that mediate the regio- and stereoselective transfer of sugars, such as the fucosyltransferases, determine cell surface-carbohydrate profiles, which is an essential interface for biological recognition processes. GTDC1 (Glycosyltransferase-like domain-containing protein 1), also known as Mat-Xa, is a 458 amino acid protein belonging to the glycosyltransferase 1 family. GTDC1 is ubiquitously expressed, with highest levels found in peripheral blood leukocytes, spleen, lung and testis.An extremely rare recessive genetic disorder, Alstr syndrome, is caused by mutations in the ALMS1 gene, which maps to chromosome 2.