Knockout mice lacking glycosyltransferases or sulfotransferases provide unequivocal evidence that the carbohydrate moieties of glycoproteins, glycolipids, and proteoglycans play essential roles in various biological phenomena such as development, the immune response, and tissue functions. Examples of abnormalities of null mutants include arrest of embryogenesis due to deletion of N-acetylglucosaminyltransferase I or glucosylceramide synthase, failure of kidney formation in heparan sulfate 2-O-sulfotransferase deficiency, suppressed antibody production in alpha-2, 6-sialyltransferase deficiency, male sterility in GM2/GD2 synthase deficiency, and abnormalities in the function and stability of myelin in galactosylceramide deficiency.