While mutations in either the PCCA or PCCB gene cause propionic acidemia (MIM# 606054), isolated methylmalonic acidemia is caused either by a genetic defect in the MUT enzyme itself (MIM# 251000, MMA mut type), or in one of the proteins (MMAA, MMAB, MMADHC) involved in the synthesis of its active cofactor, adenosylobalamin (MMA cblA type, MIM# 251100; MMA cblB type, MIM# 251110; MMA cblD-variant 2 MIM# 277410) [1]. This evidence concerns the gene MMADHC and methylmalonic acidemia.