Conditions 3-Methylcrotonyl-CoA Carboxylase Deficiency

3-methylcrotonyl-CoA carboxylase deficiency (3-MCC) is an inherited condition in which the body is unable to break down certain proteins properly. 3-MCC is considered an organic acid condition because it can lead to harmful amounts of organic acids and toxins in the body. Symptoms of 3-MCC vary. Early detection and treatment can often help children with 3-MCC lead healthy lives.

Condition Type

Organic Acid Conditions


3-methylcrotonyl-CoA carboxylase deficiency affects one out of every 36,000 to 50,000 newborns.

Also known as

  • 3-MCC
  • 3-MCC deficiency
  • 3-methylcrotonylglycinuria
  • BMCC deficiency
  • Methylcrotonyl-CoA carboxylase deficiency
  • 3-methylcrotonylglycinuria

Follow-Up Testing

Your baby’s doctor may ask you if your baby is showing any of the signs of 3-MCC (see Early Signs, below). If your baby has certain signs, your doctor may suggest starting immediate treatment.

If your baby’s newborn screening result for 3-methylcrotonyl-CoA carboxylase deficiency (3-MCC) was out of the normal range, your baby’s doctor or the state screening program will contact you to arrange for your child to have additional testing. It is important to remember that an out-of-range screening result does not necessarily mean that your child has the condition. An out-of-range result may occur because the initial blood sample was too small or the test was performed too early. However, as a few babies do have the condition, it is very important that you go to your follow-up appointment for a confirmatory test. Because the harmful effects of untreated 3-MCC can occur within days after birth, follow-up testing must be completed as soon as possible to determine whether or not your baby has the condition.

Follow-up testing will involve checking your baby’s urine and blood samples for harmful levels of acids and toxins. Certain acids and toxins build up in the body when a child has an organic acid condition, so studying the amounts of these substances in your baby’s body can help doctors determine if your baby has a condition. High levels of C5-OH acylcarnitine in the blood and organic acids in the urine might indicate that your baby has 3-MCC. Sometimes follow-up testing may also include testing a very small sample of skin.

About 3-Methylcrotonyl-CoA Carboxylase Deficiency

Early Signs

Each child with 3-methylcrotonyl-CoA carboxylase deficiency (3-MCC) has a different experience. Your child might start showing signs of 3-MCC around three to six months, three years, or even in adulthood. Some babies can have this condition and never show any signs.

Signs of 3-MCC include:

  • Poor appetite
  • Sleeping longer or more often
  • Tiredness
  • Behavior changes
  • Irritability
  • Muscle weakness (known as hypotonia)
  • Vomiting
  • Muscle tightness (also called spasticity)
  • Delayed growth
  • Developmental delays

Many of these signs may occur when your baby eats foods that their body cannot break down. They can be triggered by long periods of time without eating, illnesses, and infections.

If your baby shows any of these signs, be sure to contact your baby’s doctor immediately.


Treatments vary among children with 3-methylcrotonyl-CoA carboxylase deficiency (3-MCC). Some children never need any treatment. Your baby’s doctor will be able to determine which treatments your baby needs.

Dietary Treatment

Your baby will also need to eat often. Long periods of time without food, illnesses, and infections can trigger many of the signs mentioned in the Early Signs section.

Your baby will also need to follow a very specific diet because they cannot break down a substance in proteins called leucine. Your baby’s doctor might recommend a special leucine-free baby formula or other foods made for children with 3-MCC. These formulas will likely need to be continued through adulthood.

Supplements and Medications

L-carnitine supplements can also be helpful. L-carnitine is a natural substance that helps remove harmful waste products from the body. Your baby’s doctor can determine how much L-carnitine your child needs and write an appropriate prescription.

Expected Outcomes

Most children who are diagnosed with 3-methylcrotonyl-CoA carboxylase deficiency (3-MCC) at birth never need any special treatment. They can live healthy lives. Even children with signs of 3-MCC have typical growth and development with early and careful treatment.

If 3-MCC is not treated, babies can develop breathing problems, seizures, liver failure, and coma. This can result in brain damage or even death.


When we eat food, enzymes help break it down. Some enzymes help break down proteins into their building blocks, called amino acids. Other enzymes break down these amino acids further to their base molecules. In babies with 3-methylcrotonyl-CoA carboxylase deficiency (3-MCC), the enzyme 3-methylcrotonyl-CoA carboxylase (MCC) is not working correctly.

This enzyme’s job is to break down the amino acid leucine. If your baby has 3-MCC, they either does not make enough or makes non-working MCC enzymes. When MCC does not work correctly, your baby’s body cannot break down leucine, which causes the build-up of harmful organic acids.

3-MCC is an autosomal recessive genetic condition. This means that a child must inherit two copies of the non-working gene for 3-MCC, one from each parent, in order to have the condition. The parents of a child with an autosomal recessive condition each carry one copy of the non-working gene, but they typically do not show signs and symptoms of the condition. While having a child with 3-MCC is rare, when both parents are carriers, they can have more than one child with the condition. Learn more about autosomal recessive inheritance.

Support for 3-Methylcrotonyl-CoA Carboxylase Deficiency

Support Services

Support groups can help connect families who have a child or other family member affected with 3-methylcrotonyl-CoA carboxylase deficiency with a supportive community of people who have experience and expertise in living with the condition. These organizations offer resources for families, affected individuals, health care providers, and advocates:

Accessing Care

Work with your baby’s health care provider to determine the next steps for your child’s care. Your baby’s doctor may help you coordinate care with a doctor specializing in metabolism, a dietician who can help plan your child’s specialized diet, or other medical resources in your community. Some children with 3-methylcrotonyl-CoA carboxylase deficiency (3-MCC) have developmental delays. If you think that your baby is not meeting their developmental milestones, ask your baby’s doctor about the next steps in accessing a developmental evaluation and care.

Because 3-MCC is a genetic condition you may want to talk with a genetics specialist. A genetic counselor or geneticist can help you understand the causes of the condition, discuss genetic testing for 3-MCC, and understand what this diagnosis means for other family members and future pregnancies. Speak with your baby’s doctor about getting a referral. The Clinic Services Search Engine offered by the American College of Medical Genetics and Genomics and the Find a Genetic Counselor tool on the National Society of Genetic Counselors website are two good resources for you or your baby's health care provider to use to identify local specialists.

Families' Experiences

Damian was diagnosed with 3-MCC after his out-of-range newborn screening test was confirmed by additional testing. Read Damian’s story on the Save Babies through Screening Foundation website. You can read other family stories by visiting the Save Babies through Screening Foundation’s webpage.

Cutter is a toddler with 3-MCC. You can read about how Cutter and his family are doing on the Organic Acidemia Association (OAA) website. You can read other stories of families affected by 3-MCC by clicking on the family photos at the bottom of this page of the OAA website.

References & Sources

Visit MedlinePlus Genetics for more condition information

Visit the Screening, Technology And Research in Genetics (STAR-G) Project for condition information

ACT Sheets

Healthcare professionals can learn more about confirmatory testing by reading the American College of Medical Genetics and Genomics' Algorithm for Diagnosis and ACT Sheet, a guide for follow-up after newborn screening. You can visit this page of the ACMG website here.

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