Genes that encode the class of enzymes that catalyze the transfer of glycosyl groups to an acceptor; often another carbohydrate molecule, a protein, or inorganic phosphate. Some enzymes in this class catalyze hydrolysis - the transfer of a glycosyl group to water. Subclasses include Hexosyltransferases, Pentosyltransferases, and Sialyltransferases. (NCI) [ ]