Citrullinemia Type I

Gene : ASS1

What is Citrullinemia type I?

Citrullinemia type I is an inherited disorder that causes ammonia and other toxic substances to accumulate in the blood. This is due to a deficiency of the enzyme arginine succinate synthase. In most cases, the condition becomes evident in the first few days of life. Affected infants typically appear normal at birth, but soon afterwards experience a progressive lack of energy, poor feeding, vomiting, seizures, and loss of consciousness as ammonia builds up in their body. These medical problems are life-threatening in many cases but with appropriate treatment and dietary management, these infants survive an indeterminate period of time, sometimes without significant intellectual disability. A less common, milder form of type I citrullinemia can develop later in childhood or adulthood. Symptoms include intense headaches, partial loss of vision, problems with balance and muscle coordination, and lack of energy. Some individuals with citrullinemia type I never experience symptoms and some women may not have any medical issues until an onset of severe symptoms during pregnancy or postpartum. Citrullinemia type I is caused by pathogenic variants in the ASS1 gene.

How is Citrullinemia type I inherited?

Citrullinemia type I is inherited in an autosomal recessive manner. This type of inheritance requires the presence of two copies of a pathogenic variant in the gene for a person to have the genetic disease. Both parents must be carriers of a pathogenic variant in the gene in order to be at risk to have an affected child. The child must inherit a pathogenic variant from each carrier parent in order to be affected. There is a 1 in 4 chance that a baby will inherit two mutated copies of the gene and be affected when both parents are carriers.

What does it mean to be a carrier?

There are generally no signs or symptoms associated with being a carrier for Citrullinemia type I. However, the risk to have a child affected with this disorder is increased. Testing of reproductive partners is recommended for carriers of Citrullinemia type I.

How common is Citrullinemia type I?

Citrullinemia type I affects about 1 in 57,000 newborns worldwide, although variation is observed among different populations. Based on new born screening, it occurs in 1 in 118,543 newborns in Taiwan, 1 in 200,000 newborns in New England, 1 in 22,150 newborns in Korea, and 1 in 77,811 newborns in Austria.

Family References and Resources
Clinician References
What is Analyzed?
  • Full gene sequencing
Affected Systems




Carrier Rates
Detection Rate
Carrier Frequency
General Population 96% 1 in 119

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