Cobalamin C deficiency (cblC), caused by mutations in MMACHC, is the most common inborn error of intracellular vitamin B12 metabolism. NHGRI scientist have generated a number of Mmachc knockout mouse models. The cblC mice present with early lethality, recapitulate the neurological phenotype seen in patients, and have enabled proof of concept testing with traditional hydroxocobalamin formulations and doses. The scientist have also developed a novel combination of hydroxo- and methylcobalamin, having superior performance to traditional hydroxocobalamin only treatment. The immediate use of the results and models are to enable the formulation and testing of new cobalamin preparations (injectables) for the treatment of a large group of inborn errors of metabolism, neurological, ocular and vascular disorders.