DAG1 and muscular dystrophy-dystroglycanopathy, type A: Despite the absence of the MEP pathway inhumans, recent researchsuggests that the human homologue of IspD (HsIspD)is an essential enzyme for dystroglycan O-mannosylation.Defects or deficiency in dystroglycan O-mannosylationlead to muscular dystrophy, severe brain abnormalities, and, in somecases, the Walker–Warburg syndrome.51,52 Analysis of the crystal structure of HsIspD demonstratedstrong similarity with the active site of EcIspD,and, in extension, the whole larger subdomain.