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[Biochemistry](https://durgical.com/en/biochemistry/) · Enzyme

# UGT1A1 (bilirubin UDP-glucuronosyltransferase)

Liver UGT1A1 attaches glucuronic acid to bilirubin so it dissolves in bile; its failure causes Gilbert and Crigler-Najjar.

Updated 2026-11-01

## Key facts

- Bilirubin + UDP-glucuronic acid → bilirubin monoglucuronide, then diglucuronide + UDP.
- It is a membrane protein of the hepatocyte endoplasmic reticulum, with its active site facing the lumen.
- It is the only enzyme that conjugates bilirubin; it also glucuronidates drugs (SN-38 of irinotecan) and estrogens.
- The UGT1A1*28 variant (seven TA repeats in the promoter) lowers expression to about 30 % and causes Gilbert syndrome.

**Also called**
Bilirubin-UGT

**EC number**
[2.4.1.17](https://www.enzyme-database.org/query.php?ec=2.4.1.17)

**Genes**
UGT1A1

## Regulation

- It is induced by phenobarbital and other nuclear-receptor activators (CAR, PXR).
- At birth its activity is low and it matures over the first weeks of life.

## Inhibitors and toxins

- Atazanavir and indinavir
- Pregnanes in breast milk (proposed in breast-milk jaundice)

## Clinical notes

- Gilbert: ~30 % activity, mildly high indirect bilirubin with fasting, stress or infection; benign.
- Crigler-Najjar type 1: no activity, risk of kernicterus; type 2: very low activity that responds to phenobarbital.
- People with UGT1A1*28 have a higher risk of irinotecan toxicity (neutropenia, diarrhoea); genotyping can be done beforehand.

## Pathways

- [Heme breakdown and bilirubin](https://durgical.com/en/biochemistry/pathways/heme-breakdown-and-bilirubin/)

## Related conditions

- [Gilbert syndrome](https://durgical.com/en/conditions/gilbert-syndrome/)
- [Crigler-Najjar syndrome](https://durgical.com/en/conditions/crigler-najjar-syndrome/)
- [Neonatal jaundice](https://durgical.com/en/conditions/neonatal-jaundice/)

## Related

- [Bilirubin](https://durgical.com/en/biochemistry/molecules/bilirubin/)
- [Biliverdin reductase](https://durgical.com/en/biochemistry/enzymes/biliverdin-reductase/)
- [Heme oxygenase](https://durgical.com/en/biochemistry/enzymes/heme-oxygenase/)

## Test yourself

Think of the answer, then tap the question to see it.

## Which UGT1A1 variant most often causes Gilbert syndrome?

UGT1A1*28: seven TA repeats in the promoter that lower enzyme expression.

## Which type of Crigler-Najjar responds to phenobarbital, and why?

Type 2: some enzyme remains and phenobarbital induces it; in type 1 there is no enzyme to induce.

## Which anticancer drug is more toxic in people with UGT1A1*28?

Irinotecan: its active metabolite SN-38 is cleared by UGT1A1 glucuronidation.

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Educational information, not medical advice. For any symptom, see a health professional.

Sources: [UGT1A1: UDP-glucuronosyltransferase 1A1 (human)](https://www.uniprot.org/uniprotkb/P22309/entry) (UniProt, 2026); [UGT1A1 gene: MedlinePlus Genetics](https://medlineplus.gov/genetics/gene/ugt1a1/) (MedlinePlus, US National Library of Medicine, 2026); [Gilbert syndrome: MedlinePlus Genetics](https://medlineplus.gov/genetics/condition/gilbert-syndrome/) (MedlinePlus, US National Library of Medicine, 2026); [Crigler-Najjar syndrome: MedlinePlus Genetics](https://medlineplus.gov/genetics/condition/crigler-najjar-syndrome/) (MedlinePlus, US National Library of Medicine, 2026).

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