MMACHC (metabolism of cobalamin associated C) is a cytosolic chaperone essential for intracellular vitamin B12 (cobalamin) metabolism. Its primary function is catalyzing the reductive decyanation of cyanocobalamin (dietary B12) to cob(II)alamin using FAD/FMN cofactors and NADPH 1. This conversion is necessary because dietary cobalamin cannot be utilized directly; the resulting cob(II)alamin serves as a precursor for generating two active coenzyme forms: methylcobalamin (MeCbl) for methionine synthesis and 5'-deoxyadenosylcobalamin (AdoCbl) for the TCA cycle 1. MMACHC also functions as a glutathione transferase, dealkylating MeCbl and AdoCbl into cob(I)alamin using glutathione 1. The protein operates within a multiprotein complex including MMADHC, MTRR, and MTR to safely shuttle cobalamin toward methionine synthase 2. Mutations in MMACHC cause cblC disease, the most common inborn error of cobalamin metabolism 3. This results in combined methylmalonic aciduria and homocystinuria—impaired conversion of dietary B12 to active forms causes accumulation of methylmalonic acid and homocysteine while decreasing methionine synthesis 3. Clinical presentation is heterogeneous: early-onset patients (within first year) exhibit severe multisystem disease affecting neurological, ocular, hematological, renal, and cardiac systems, while late-onset patients show milder progressive neurological symptoms 3. Treatment combines vitamin B12 supplementation, betaine, and folic acid to improve metabolic parameters, though long-term outcomes remain unsatisfactory, particularly for early-onset cases 3.