What you carry at this position in your DNA and what research says about it.
Bilirubin and Drug Clearance: 8/10 evidence, 6 publications.
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Sample genotype (not yours): C/C
Typical Bilirubin and Drug Clearance
Normal enzyme levels ensure efficient processing of cellular waste and standard medications. This genotype protects against Gilbert syndrome and reduces the risk of severe side effects from specific therapies.
What this genotype means: Efficient bilirubin processing; Standard clearance of irinotecan; Low risk of drug-induced jaundice.
Sample genotype (not yours): C/CAT
Intermediate Bilirubin and Drug Clearance
Carrying one copy of the insertion slightly reduces liver enzyme activity. While usually asymptomatic, this profile can lead to mild bilirubin fluctuations and intermediate drug sensitivity.
What this genotype means: Slightly reduced liver conjugation capacity; Occasional mild bilirubin spikes under stress; Intermediate sensitivity to specific medications.
Sample genotype (not yours): CAT/CAT
Reduced Clearance and Gilbert Syndrome Risk
Two copies of the insertion significantly lower liver enzyme activity, causing Gilbert syndrome. This benign condition leads to elevated bilirubin and increases the risk of toxicity from specific medications.
What this genotype means: Frequent mild jaundice from Gilbert syndrome; High risk of severe irinotecan toxicity; High risk of atazanavir-induced hyperbilirubinemia.
Gene function and biological role
Gene Function: The UGT1A gene family encodes enzymes responsible for glucuronidation, a critical liver process that makes toxins, drugs, and cellular waste water-soluble for excretion [1]. Among these, the UGT1A1 enzyme is the primary driver of bilirubin clearance, breaking down the yellow byproduct of old red blood cells [2]. When these enzymes function at a reduced capacity, unconjugated bilirubin builds up in the bloodstream [3]. This gene cluster also plays a major role in metabolizing several clinical medications, meaning genetic variations directly impact drug clearance rates and toxicity risks [1].
Variant Impact: This genetic variant represents an insertion of an extra thymine-adenine (TA) pair in the promoter region of the UGT1A gene cluster, commonly referred to as the UGT1A1*28 allele [2]. The reference sequence contains six TA repeats, whereas this variant contains seven repeats [2]. This extra pair alters the TATA box promoter element, significantly reducing the transcription and overall expression of the glucuronosyltransferase enzyme [2]. Consequently, individuals carrying this insertion experience a substantial decrease in their liver's capacity to process bilirubin and clear specific medications [1].
Key Insight: An extra pair of DNA letters in this gene's promoter reduces liver enzyme activity by up to seventy percent, turning a routine cellular cleanup process into a visible but harmless yellowing of the skin.
Location: UGT1A1. Chromosome 2 · 2q37.1 · 233.8 Mb
Estimated prevalence by population
Global 43.1%; European 46.3%; African 36.3%; East Asian 76.3%; South Asian 35.1%; Americas 46.3%; Ashkenazi 38.3%; Finnish 34.0%; Middle Eastern 45.8%; Amish 33.4%.
Estimated CC genotype frequency.
Global 44.6%; European 43.5%; African 47.9%; East Asian 22.1%; South Asian 48.3%; Americas 43.5%; Ashkenazi 47.2%; Finnish 48.6%; Middle Eastern 43.7%; Amish 48.8%.
Estimated CCAT genotype frequency.
Global 12.3%; European 10.2%; African 15.8%; East Asian 1.6%; South Asian 16.6%; Americas 10.2%; Ashkenazi 14.5%; Finnish 17.4%; Middle Eastern 10.4%; Amish 17.8%.
Estimated CATCAT genotype frequency.
Showing effects for C/C genotype
Rare Genetic Disorders
Inherited Metabolic Disorders
Gilbert Syndrome. Evidence: 5/10. Source: PMID 7565971. Typical Risk.
Pharmacogenomics
Chemotherapy Response
Irinotecan Toxicity. Evidence: 8/10. Source: PMID 28520360. Normal Clearance.
Antimicrobial & Antiviral Response
Atazanavir Response. Evidence: 8/10. Source: PMID 28520360. Normal Clearance.
Organ Systems
Liver Function & Disease
Newborn Jaundice. Evidence: 5/10. Source: PMID 10190918. Typical Risk.
Showing effects for C/CAT genotype
Gilbert Syndrome. Evidence: 5/10. Source: PMID 7565971. Slightly Elevated.
Irinotecan Toxicity. Evidence: 8/10. Source: PMID 28520360. Intermediate Risk.
Atazanavir Response. Evidence: 8/10. Source: PMID 28520360. Intermediate Risk.
Newborn Jaundice. Evidence: 5/10. Source: PMID 10190918. Slightly Elevated.
Showing effects for CAT/CAT genotype
Gilbert Syndrome. Evidence: 5/10. Source: PMID 7565971. Highly Elevated.
Irinotecan Toxicity. Evidence: 8/10. Source: PMID 28520360. High Toxicity Risk.
Atazanavir Response. Evidence: 8/10. Source: PMID 28520360. High Jaundice Risk.
Newborn Jaundice. Evidence: 5/10. Source: PMID 10190918. Elevated Risk.
What this genotype means for the people who carry it
Sample genotype: CC (Homozygous)
Efficient liver processing is maintained by two copies of the standard promoter sequence, which contains six thymine-adenine repeats [1]. Bilirubin is conjugated and excreted smoothly, keeping circulating levels within the normal range and preventing the yellowing of the skin or eyes known as jaundice [2]. Standard clearance rates also apply to clinical drugs like the chemotherapy agent irinotecan and the HIV medication atazanavir [3]. Because the liver metabolizes these compounds at a typical pace, the risk of severe treatment-limiting toxicities, such as profound neutropenia or drug-induced hyperbilirubinemia, remains low [3]. No special dietary or medical adjustments are typically required for this genetic profile.
Efficient bilirubin processing; Standard clearance of irinotecan; Low risk of drug-induced jaundice.
Sample genotype: CCAT (Heterozygous)
Enzyme expression is moderately reduced due to the presence of one copy of the seven-repeat promoter sequence [1]. While the liver generally compensates well enough to keep baseline bilirubin levels near normal, mild elevations can occur during periods of fasting, dehydration, or physical stress [2]. This intermediate metabolic rate also affects how the body processes certain medications, such as irinotecan and atazanavir [3]. Although the risk is lower than in those with two copies of the variant, there is still a slightly elevated chance of experiencing drug-related side effects or mild jaundice during treatment [3]. Knowing this status helps clinicians monitor drug levels more closely if these specific therapies are ever prescribed.
Slightly reduced liver conjugation capacity; Occasional mild bilirubin spikes under stress; Intermediate sensitivity to specific medications.
Sample genotype: CATCAT (Homozygous)
Hepatic glucuronidation capacity is reduced by up to seventy percent because of two copies of the seven-repeat promoter sequence [1]. This genetic profile is the primary cause of Gilbert syndrome, a benign, lifelong condition characterized by intermittent, mild jaundice [2]. Yellowing of the skin or eyes is harmless and typically triggered by fasting, dehydration, illness, or strenuous exercise [3]. However, this slow metabolism becomes highly clinically relevant when taking specific medications, particularly the chemotherapy drug irinotecan and the antiviral atazanavir [4]. Because the body cannot clear these drugs efficiently, standard doses can lead to severe, potentially dangerous toxicities like neutropenia or extreme jaundice [4].
Frequent mild jaundice from Gilbert syndrome; High risk of severe irinotecan toxicity; High risk of atazanavir-induced hyperbilirubinemia.
Variants with shared genetic architecture
rs6742078 · UGT1A1 Gene · Chr 2 · Gallstone Risk
rs887829 · UGT1A1 Gene · Chr 2 · Bilirubin and Heart Health
rs10929302 · UGT1A1 Gene · Chr 2 · Drug and Bilirubin Clearance
Irinotecan Therapy and UGT1A1 Genotype.
Content updated August 2026.
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