Methylmalonyl-CoA epimerase, mitochondrial（MCEE）is an enzyme which belongs to the glyoxalase I family. It converts (S)-methylmalonyl-CoA to the (R) form, catalyses the following chemical reaction: (R)-methylmalonyl-CoA (S)-methylmalonyl-CoA. It plays an important role in the catabolism of fatty acids with odd-length carbon chains. This protein deficiency is an autosomal recessive inborn error of AA metabolism, involving valine, threonine, isoleucine and methionine. This organic aciduria can appear in the neonatal period with life-threatening metabolic acidosis, hyperammonemia, feeding difficulties, pancytopenia and coma.