MMUT and Methylmalonic aciduria: Depending on the enzymatic block, three major organic acidurias have long been distinguished: isovaleric aciduria (IVA) designates an isovaleryl-coA deshydrogenase defect on the leucine degradation pathway secondary to mutation in the IVD gene, propionic aciduria (PA) results from a defect in propionyl-coA carboxylase encoded by the PCCA or PCCB genes, and methylmalonic aciduria (MMA) is caused by mutations in the MUT gene, encoding the methylmalonyl-CoA mutase (MCM), or more rarely in genes encoding the coenzyme adenosylcobalamin of MCM.