The MMAB gene has been mapped to chromosome 12q24.11, where it encodes an enzyme of 250 amino acids that plays a role in the formation of a complex called adenosylcobalamin (AdoCbl). The MMAB enzyme is active in the mitochondria, where it catalyzes the final step in the conversion of vitamin B(12) into adenosylcobalamin (AdoCbl). The AdoCbl complex plays an important role in the normal function of an enzyme called methylmalonyl CoA mutase. Defects in this enzyme are the cause of methylmalonic acidemia, an autosomal recessive disease characterized by feeding difficulties, developmental delay, and long-term health problems.