MMAB (metabolism of cobalamin associated B) encodes ATP:cobalamin adenosyltransferase, which catalyzes the conversion of cob(I)alamin to adenosylcobalamin (AdoCbl), the essential coenzyme for methylmalonyl-CoA mutase 1. This enzyme functions in the final step of vitamin B12 activation, directly delivering AdoCbl to MUT in an ATP-dependent manner 2. MMAB mutations cause cblB-type methylmalonic aciduria, an autosomal-recessive propionate metabolism disorder presenting with variable clinical severity correlating with specific pathogenic variants 23. Beyond its classical role in cobalamin metabolism, MMAB participates in cholesterol homeostasis regulation; hepatic MMAB expression is modulated by dietary cholesterol through SREBP2, and MMAB knockdown increases propionate and methylmalonic acid levels, which inhibit cholesterol biosynthesis and enhance LDL-receptor expression 4. Population studies indicate MMAB genetic variants associate with HDL-cholesterol levels and coronary heart disease risk in Han Chinese populations 5. Additionally, MMAB appears implicated in schizophrenia susceptibility through mitochondrial dysfunction pathways, with increased MMAB expression associated with elevated schizophrenia risk 6. During embryonic development, MMAB shows tissue-specific expression in organogenesis, suggesting roles beyond isolated metabolic functions 7.