Crystallization of DPAGT1 in its apo state, when bound to UDP-GlcNac or tunicamycin and functional studies of DPAGT1 enzymatic activity showed that mutations the cause the multisystem disorder (CDG1j) are either null (frameshift or nonsense mutations) or affects catalytic activity, whereas those causing CMS tend to be located further from the catalytic site and may reduce protein expression/dimerization or modestly decrease catalytic activity. This evidence concerns the gene DPAGT1 and DPAGT1-congenital disorder of glycosylation.