The UGT1A gene family helps the liver change bilirubin, a yellow pigment from normal red blood cell breakdown, and many medicines into forms the body can remove. If this system works less well, bilirubin can build up and cause yellowing of the skin or eyes, and some drugs may stay in the body longer or cause more side effects. Variants in the UGT1A1 part of this gene family are linked to Gilbert syndrome and, less often, Crigler-Najjar syndrome. This gene can play a role in how your body responds to irinotecan, a cancer medicine.
What is a gene?
Genes are sections of DNA that carry instructions your cells use to make molecules or guide processes in the body. Some genes hold the instructions for producing proteins, the working parts within cells. Others mainly help regulate when certain processes happen, where they happen, and how active they are. Together, genes affect how the body develops and works, and they can also shape how you respond to food, stress, or medications. To explore this topic in more detail, you can find more in the LearnHub.
Overview
UGT1AGene function
What if the gene works
Your body uses the UGT1A gene to help process bilirubin, a yellow pigment made when red blood cells break down. It also helps the liver make some hormones, medicines, and other compounds easier to remove.
Risk
What if the gene does not work
Variants in the UGT1A gene can reduce bilirubin processing, leading to jaundice that ranges from mild, intermittent Gilbert syndrome to the much more severe Crigler-Najjar syndrome. Milder and more severe variants can also change how some medicines are handled.
Treatment
What does that mean for your treatment
Pathogenic variants in the UGT1A gene can guide treatment by showing whether some medicines are likely to help. With partial loss of enzyme activity, phenobarbital can lower bilirubin and is often part of management. With complete loss of activity, phenobarbital is not effective, so care may rely on intensive phototherapy and, in some cases, liver transplantation.
Gene function
What if the gene works
Your body needs a steady way to package used chemicals so they can leave safely. The UGT1A gene group helps make enzymes that attach a small chemical tag to bilirubin, some medicines, hormones, and other compounds. This makes them easier to move out through bile or urine.
UGT1A enzymes attach a small chemical tag to certain substances in your body. This tag makes them easier to dissolve and easier to remove through bile or urine.
One key role of the UGT1A gene group is helping the liver process bilirubin, a yellow substance made when old red blood cells are broken down. After tagging, bilirubin can pass into bile and leave the body.
These enzymes also help your body handle many medicines and everyday chemicals. This is part of what the UGT1A gene does in the liver and gut to support normal chemical clearance.
UGT1A enzymes help process some hormones and compounds from food or the environment. By changing them into forms the body can remove, they support normal day to day balance.
What is a variant?
A genetic variant is a minor difference in the DNA sequence within a gene. These differences are common and help shape normal human diversity, including differences in appearance, metabolism, and the way the body responds to environmental influences. Many variants do not cause any noticeable effect and are simply part of natural genetic variation. Sometimes, though, a variant changes how a gene works more significantly and may contribute to or cause a genetic condition.
Genetic variants
What if the gene does not work
Variants in the UGT1A gene can change the shape, stability, or amount of related enzymes, so cells clear bilirubin, a yellow waste product, less effectively. Milder variants often leave some activity, while more severe variants can greatly reduce or nearly stop this step, which may influence how early jaundice appears and how marked it is. This is why how UGT1A gene variants affect bilirubin processing can differ from one person to another.
How variants are linked to conditions
Pathogenic variants in the UGT1A gene are linked to conditions that affect bilirubin levels, especially Gilbert syndrome and Crigler-Najjar syndrome types 1 and 2. In practice, this means UGT1A gene variants linked to conditions often come to attention after mild or persistent jaundice, with yellowing of the skin or eyes. Gilbert syndrome is usually mild, while Crigler-Najjar syndrome can be much more significant and may confirm an inherited diagnosis.
A variant is not necessarily a diagnosis
Gene variants are a normal feature of human biology and are common across the general population. Most have no direct impact on health and simply reflect natural human diversity. For many genes, carrying a variant associated with a condition indicates a possible link rather than a definite diagnosis, especially when several genes and environmental factors are involved. In some situations, however, a single well-characterized variant in a particular gene is enough to confirm a diagnosis.
Are there things that can increase or change my risk?
For people with changes in UGT1A, risk is often shaped by the specific change you carry and by everyday triggers. Fasting, dehydration, illness, heavy exercise, stress, lack of sleep, and some medicines can make episodes of higher bilirubin and yellowing of the eyes or skin more likely, especially in people with Gilbert syndrome.
Inheritance of UGT1A variants
Variants in the UGT1A gene linked to inherited high bilirubin conditions are usually passed down in an autosomal recessive pattern. This means a child typically inherits one changed copy from each parent, while parents with one copy are often carriers and usually do not have the full condition. When people look up UGT1A inheritance, this is the pattern most often described for conditions such as Gilbert syndrome and Crigler-Najjar syndrome. Each pregnancy between two carriers has a 25% chance of producing a child with the condition, a 50% chance of producing a carrier, and a 25% chance of producing a child without the variant.
What does my UGT1A result mean for my family?
For inherited UGT1A-related conditions such as some causes of ongoing high bilirubin, the result is usually passed through families in a recessive pattern. This means brothers and sisters may also be affected or may be carriers, and parents are often carriers even if they have no symptoms. Children of someone with two changed copies will usually be carriers, unless the other parent also carries a UGT1A change.
Genetic information
Genetic testing
How you find out about variants
Genetic testing is how you find out about variants in the UGT1A gene. A lab usually uses a small blood or saliva sample to read the DNA code and look for specific changes in the gene. Results may show no variant, a known variant, or a change whose meaning is still being clarified.
When should I consider getting a genetic test?
Consider genetic testing related to the UGT1A gene if you or your child has repeated unexplained yellowing of the skin or eyes, high bilirubin, or a family history of inherited bilirubin conditions. Testing may also be considered before treatment with certain medicines, especially irinotecan, because some UGT1A changes can raise the chance of side effects.
A genetic result is only one part of your health story
A genetic finding can provide useful insight, but what it can tell you depends on the condition and on the kind of variant involved. Over time, many factors, including lifestyle, environment, age, and medical history, influence how the body functions. For some conditions, particularly those shaped by many genes, genetic information offers context rather than certainty and makes the most sense when considered with other parts of care. In other situations, a genetic result can give a clear answer. In areas like medication response, it can also help put personal experiences in context.
Pharmacogenetics
How variants shape your response to medicine
Genetic variation can affect how your body handles certain medicines. Some people break down or clear a drug more slowly or more quickly, which can change how well it works or how likely side effects are. These differences are common and help explain why the same treatment can affect one person differently from another. Genetics is only one part of the picture, alongside age, overall health, and other medicines.
What is Pharmacogenetics?
Pharmacogenetics is the area of study that examines how genetic differences affect the way people respond to medicines. It focuses on links between genes and medications to help explain why the same treatment can work differently in different people. For many drugs, genes influence either how much effect a medicine has or how the body handles it. This information can support more informed, more personalized treatment decisions. To explore this subject further, more information is available in the LearnHub.
Drugs influenced by the UGT1A gene
When should I seek medical advice?
Talk with your doctor before starting or changing treatment with irinotecan or atazanavir if you know you have a change in the UGT1A1 gene. UGT1A1 can affect how your body handles these medicines and may raise the chance of side effects, including yellowing of the skin or eyes with atazanavir. It is also worth bringing up if you had unusually strong side effects with these treatments in the past.
History
Since the earliest days of medicine, efforts have been made to understand inherited patterns in unexplained jaundice. In the mid 20th century, doctors began distinguishing milder inherited jaundice, later called Gilbert syndrome, from the more severe Crigler Najjar syndrome, which set the stage for work on the UGT1A gene. From these first observations, research expanded steadily. In the 1990s, scientists clarified that the UGT1A gene region is an unusual cluster that can produce several related enzymes, and they linked specific variants to bilirubin disorders. That discovery also shaped cancer drug research, because UGT1A1 variants were later tied to irinotecan side effects. Once considered mainly a liver genetics topic, UGT1A became an important example in pharmacogenetics, the study of how genes influence medicine response.