Conditions Holocarboxylase Synthetase Deficiency

Holocarboxylase synthetase deficiency (MCD) is a condition in which the body is unable to break down proteins and carbohydrates. People with this condition have trouble using biotin, a vitamin that helps turn certain carbohydrates and proteins into energy for the body. Holocarboxylase synthetase deficiency is considered an organic acid condition because it can lead to a harmful buildup of organic acids and toxins in the body. Early detection and treatment often can prevent the severe outcomes of MCD.

Condition Type

Organic Acid Conditions


Holocarboxylase synthetase deficiency is estimated to affect one out of every 87,000 people.

Also known as

  • MCD
  • HCSD
  • Holocarboxylase deficiency
  • HLCS deficiency
  • Early-onset multiple carboxylase deficiency
  • Multiple carboxylase deficiency
  • Neonatal form of MCD
  • Multiple co-A carboxylase deficiency

Follow-Up Testing

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

If your baby’s newborn screening result for holocarboxylase synthetase deficiency (MCD) was out of the normal range, your baby’s doctor or the state screening program will contact you to arrange for your baby 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, a few babies do have the condition, so it is very important that you go to your follow-up appointment for a confirmatory test. Because the harmful effects of untreated MCD can occur soon 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 levels of these substances in your baby’s body can help doctors determine if your child has a condition. High levels of C5-OH acylcarnitine in the blood and organic acids in the urine might be signs that your baby has MCD. Your baby’s doctor may also test to see if your baby’s body is making and using biotin. In some cases, follow-up testing may include testing a very small sample of skin.

About Holocarboxylase Synthetase Deficiency

Early Signs

Signs of holocarboxylase synthetase deficiency (MCD) begin in infancy, usually in the first few days of life. These signs include:

  • Poor appetite
  • Vomiting
  • Sleeping longer and more often
  • Tiredness
  • Irritability
  • Weak muscle tone (known as hypotonia)
  • Peeling skin rash
  • Hair loss (called alopecia)

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.


Babies with holocarboxylase synthetase deficiency (MCD) need to take biotin supplements to replace the natural biotin that is not working correctly. Biotin is a type of vitamin that your body needs to digest food. These supplements can prevent your baby from experiencing many of the signs of MCD mentioned in the Early Signs section. In some cases, biotin supplements can reverse any signs that are already present.

This is usually the only treatment your baby will need, but it is important to start treatment early to avoid any life-threatening signs and symptoms. Biotin treatment will continue throughout your child’s life. Your baby’s doctor will need to write a prescription for these supplements.

Expected Outcomes

If your baby takes biotin supplements, they can live a healthy life and have typical development. With biotin treatment, your child will not need restricted diets or activities.

If holocarboxylase synthetase deficiency is not treated, children can develop seizures or permanent brain damage.


When we eat food, enzymes help break it down for energy. Vitamins are also important to help our bodies break down food. In holocarboxylase synthetase deficiency (MCD), the enzyme holocarboxylase synthetase is not working correctly.

This enzyme’s job is to attach a vitamin, called biotin, to other enzymes. These other enzymes need biotin in order to break down food.

MCD occurs when your baby’s body is not making enough or making non-working holocarboxylase sythetase. When holocarboxylase synthetase is not working correctly, food cannot be broken down completely. This causes harmful substances to build up in the body.

MCD is an autosomal recessive genetic condition. This means that a child must inherit two copies of the non-working gene for MCD, 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 MCD 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 Holocarboxylase Synthetase Deficiency

Support Services

Support groups can help connect families who have a child or other family member affected with holocarboxylase synthase 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, healthcare providers, and advocates.

Accessing Care

Work with your baby’s doctor to determine the next steps for your child’s care. Your baby’s doctor may help you coordinate care with a physician who specializes in metabolism, a dietician, or other medical resources in your community. Some children with holocarboxylase synthase deficiency (MCD) 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 MCD 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 MCD, and understand what this diagnosis means for other family members and future pregnancies. Speak with your baby’s doctor about getting a referral. The Genetic Clinic Search Engine offered by the American College of Medical Genetics and Genomics (ACMG) and the Find a Genetic Counselor tool on the National Society of Genetic Counselors (NSGC) website are two good resources for you or your baby's health care provider to use to identify local specialists.

Families' Experiences

At this time, we do not have any family stories for this particular condition. If your family is affected by holocarboxylase synthetase (MCD), we would love to hear from you, so that others may benefit from your story.

Each family has a unique experience and each condition is very different. However, you may find it useful to read the stories of families affected by other conditions, such as biotinidase deficiency, that may have similar evaluations and treatment. You can read stories of families affected by biotinadase deficiency on this support group website.

References & Sources

Visit MedlinePlus Genetics for more information

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