Methylmalonyl-Coenzyme A mutase deficiency (MCM deficiency) is a type of methylmalonic acidemia caused by having too little methylmalonyl-CoA mutase. Methylmalonyl-CoA mutase (MCM) is one of the special proteins (enzymes) needed to breakdown certain amino acids found in the food we eat. It is needed to breakdown certain fats too. When the amino acids and fats are not broken down normally, substances which are harmful to the body (including methylmalonic acid) build up and can damage the nervous system, kidneys and other organs. Symptoms of MCM deficiency usually begin in infancy or early childhood and may include tiredness (fatigue), vomiting, dehydration, weak muscle tone (hypotonia), fever, breathing trouble, frequent illnesses and infections, and increased bleeding and bruising. Long term complications include developmental delay, intellectual disability, enlarged liver (hepatomegaly), chronic kidney disease, inflammation of the pancreas (pancreatitis), and in severe cases coma and death. Methylmalonyl-Coenzyme A mutase deficiency (MCM deficiency) is caused by changess in the MUT gene which can cause no enzyme to be produced (MUT0) or less than normal amounts of the enzyme to be made (MUT-). The more working enzyme that is made, the less severe the symptoms will be. MCM deficiency is inherited in an autosomal recessive manner. Diagnosis is made based on symptoms, special blood tests and genetic testing. Unlike some types of methylmalonic acidurias, B12 supplements are not helpful.