MMAB encodes ATP:cobalamin adenosyltransferase, which catalyzes the final step of vitamin B12 activation by converting cob(I)alamin to adenosylcobalamin (AdoCbl), the essential cofactor for methylmalonyl-CoA mutase in propionate metabolism. The enzyme binds ATP and cobalamin through distinct active sites and directly transfers AdoCbl to its target enzyme in an ATP-gated mechanism 1. Biallelic MMAB variants cause cblB-type methylmalonic aciduria, an autosomal-recessive disorder characterized by impaired propionate metabolism and elevated methylmalonic acid 1. Clinical severity correlates with variant type: truncating variants (e.g., p.Arg234*) tend to show cobalamin responsiveness and later disease onset, while recurrent missense variants (p.Arg186Trp, p.Arg191Trp) typically cause early-onset disease with limited cobalamin response 1. At the population level, gnomAD v4.1 classifies this gene as LoF-tolerant (LOEUF=0.87); this is distinct from clinical pathogenicity in disease contexts, where 109 pathogenic+likely pathogenic ClinVar variants have been reported. Beyond propionate metabolism, MMAB has emerged as a regulatory node in cholesterol homeostasis: SREBP2-mediated changes in MMAB expression modulate intracellular cholesterol levels and LDL-cholesterol uptake, with knockdown increasing propionic and methylmalonic acid accumulation to suppress cholesterol biosynthesis 2. Genetic associations link MMAB polymorphisms to dyslipidemia and coronary heart disease risk in Han Chinese individuals, with favorable alleles conferring reduced CHD susceptibility 3. Recent evidence also suggests associations between increased MMAB levels and schizophrenia risk 4.