Methylmalonic acidemia (MMAemia) is an inborn error of organic acid metabolism characterized by the accumulation of toxic metabolites—including methylmalonic acid (MMA), 2-methylcitric acid (2-MCA), propionic acid (PA), homocysteine (Hcy), ammonia, and lactate—due to defects in methylmalonyl-CoA mutase or impaired cobalamin metabolism. This evidence concerns the gene MMUT and methylmalonic acidemia.