In the 15 years since the demonstration that HEX A is the defective enzyme in patients with TSD, intensive efforts in many laboratories have revealed much about the HEX group of enzymes. In contradistinction to the two isozymes described by Robinson and Stirling [1968], we now know that there are several different species. They include the products of at least three genes which code for the alpha and beta polypeptides as well as for an enzyme that we have called HEX D. The different species of HEX found in human tissues and fluids include significant amounts of larger, unprocessed polypeptides as well as mature enzyme. Thus the HEX A of serum (HEX AS) is a more primitive form of the enzyme than that found in lysosomes. The role of biosynthesis in the formation of multiple species of HEX is not confined to the polypeptide chains of the enzyme. All lysosomal enzymes are glycosylated and HEX is not an exception. The carbohydrate side-chains are essential to the packaging process that places HEX in the lysosome. Carbohydrates on lysosomal HEX species clearly differ from those on HEX in serum. Characterization of HEX oligosaccharides is still in the preliminary stages. Many minor species of HEX have been described. The more important ones are the intermediate isozymes (HEX Is). In tissues the HEX Is appear to contain mixtures of processed and unprocessed alpha and beta polypeptides. In serum, on the other hand, they contain unprocessed beta chains and differ in the carbohydrate side-chains. Most species of HEX show microheterogeneity. Native, mature HEX B separates into several bands on isoelectric focusing. The nature of this microheterogeneity has not been defined. Clear differences have been described, however, between the two chains in the beta subunit. The chains are always united in non-random fashion and are probably derived by cleavage of a single gene product. Studies of hydrolytic activity have been interesting. Like other lysosomal enzymes, HEX A requires a specific protein activator for optimum activity. This small polypeptide has been partially characterized but its mode of action is as yet unclear. Defects in activator synthesis lead to a form of GM2 ganglioside storage disease. Clinically many different phenotypes have been identified which appear to result from defects in each of the HEX genes. The differences between the defects have not been characterized in molecular terms.(ABSTRACT TRUNCATED AT 400 WORDS)