What you carry at this position in your DNA and what research says about it.
Chemotherapy Toxicity Risk: 7/10 evidence, 10 publications.
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Sample genotype (not yours): T/T
Typical Chemotherapy Clearance and Lower Toxicity Risk
Efficient liver transport supports the normal clearance of medications like methotrexate. This reduces the likelihood of drug accumulation and associated side effects.
What this genotype means: Typical clearance of methotrexate; Lower risk of severe mouth sores during chemotherapy; Slightly higher risk of mercaptopurine gastrointestinal toxicity.
Sample genotype (not yours): C/C
Slowest Chemotherapy Clearance and Highest Toxicity Risk
Carrying two copies of the C allele significantly reduces liver transporter function. This leads to prolonged drug exposure and a higher risk of side effects.
What this genotype means: Slowest clearance of methotrexate; Highest risk of severe chemotherapy-induced toxicities; Altered lipid metabolite levels.
Sample genotype (not yours): T/C
Slower Chemotherapy Clearance and Higher Toxicity Risk
Carrying one copy of the C allele is linked to reduced liver transporter activity. This can lead to slower processing of certain chemotherapy drugs.
What this genotype means: Slower clearance of methotrexate; Increased risk of chemotherapy-induced mouth sores; Altered blood metabolite levels.
Gene function and biological role
Gene Function: The SLCO1B1 gene encodes a membrane-bound transporter protein called OATP1B1, which is primarily active in the liver. This transporter facilitates the uptake of various compounds from the blood into liver cells for clearance and excretion [1]. Key substrates of this transporter include bilirubin, bile acids, and several widely used medications [2]. Genetic variations in this gene can significantly impair transporter function, leading to elevated drug levels in the blood and an increased risk of adverse side effects [1].
Variant Impact: This genetic variant is located within an intron of the SLCO1B1 gene and serves as a strong marker for transporter activity. It is inherited in close linkage with functional coding variants that directly reduce the transporter's capacity to move compounds into liver cells [1]. Consequently, this specific marker is highly associated with altered clearance rates of therapeutic drugs [3]. Testing for this variant helps identify individuals who may experience slower drug elimination and higher systemic exposure [4].
Key Insight: Slower clearance of chemotherapy drugs can lead to severe side effects like painful mouth sores, but knowing your genotype helps clinicians tailor monitoring strategies.
Location: SLCO1B1. Chromosome 12 · 12p12.1 · 21.2 Mb
Estimated prevalence by population
Global 66.8%; European 68.7%; African 70.8%; East Asian 30.2%; South Asian 83.1%; Americas 67.3%; Ashkenazi 68.6%; Finnish 46.1%; Middle Eastern 65.3%; Amish 86.9%.
Estimated TT genotype frequency.
Global 29.3%; European 28.4%; African 26.7%; East Asian 49.5%; South Asian 16.1%; Americas 29.5%; Ashkenazi 28.4%; Finnish 43.6%; Middle Eastern 31.0%; Amish 12.7%.
Estimated TC genotype frequency.
Global 3.9%; European 2.9%; African 2.5%; East Asian 20.3%; South Asian 0.8%; Americas 3.2%; Ashkenazi 2.9%; Finnish 10.3%; Middle Eastern 3.7%; Amish 0.5%.
Estimated CC genotype frequency.
Showing effects for T/T genotype
Pharmacogenomics
Chemotherapy Response
Chemotherapy Toxicity. Evidence: 7/10. Source: PMID 19901119. Typical Clearance.
Immunosuppressant & Biologic Response
Mercaptopurine Toxicity. Evidence: 5/10. Source: PMID 22846425. Increased Risk.
Organ Systems
Liver Function & Disease
Bilirubin Levels. Evidence: 3/10. Source: PMID 23281178. Typical Levels.
Showing effects for C/C genotype
Chemotherapy Toxicity. Evidence: 7/10. Source: PMID 19901119. Slowest Clearance.
Mercaptopurine Toxicity. Evidence: 5/10. Source: PMID 22846425. Typical Risk.
Bilirubin Levels. Evidence: 3/10. Source: PMID 23281178. Altered Levels.
Showing effects for T/C genotype
Chemotherapy Toxicity. Evidence: 7/10. Source: PMID 19901119. Slower Clearance.
What this genotype means for the people who carry it
Sample genotype: TT (Homozygous)
Efficient transport of organic anions ensures that medications are cleared from the bloodstream at a typical rate [1]. Clinical studies show that this genotype is associated with normal elimination kinetics for high-dose methotrexate therapy [2]. Because the drug does not linger in the body at toxic levels, the risk of severe side effects like mucositis is significantly lower compared to carriers of the C allele [3]. However, some research suggests a slightly higher sensitivity to gastrointestinal side effects when taking mercaptopurine [4]. Overall, this genetic profile represents the standard baseline for drug processing in the liver.
Typical clearance of methotrexate; Lower risk of severe mouth sores during chemotherapy; Slightly higher risk of mercaptopurine gastrointestinal toxicity.
Sample genotype: CC (Homozygous)
Significantly impaired liver transport leads to the slowest clearance rates of medications like methotrexate [1]. Individuals with two copies of the C allele experience prolonged high concentrations of the drug in their blood [2]. This slow elimination is strongly linked to an elevated risk of severe toxicities, including profound mucosal inflammation and liver strain [3]. Clinical studies highlight the importance of monitoring drug levels in patients with this genotype to avoid severe complications [4]. While this variant is less common in some populations, its impact on drug processing is highly pronounced.
Slowest clearance of methotrexate; Highest risk of severe chemotherapy-induced toxicities; Altered lipid metabolite levels.
Sample genotype: TC (Heterozygous)
Reduced transporter activity in the liver slows down the clearance of specific medications from the bloodstream [1]. Having one copy of the C allele is associated with higher plasma concentrations of methotrexate during treatment [2]. This prolonged exposure to the drug increases the risk of developing painful complications such as mucositis and gastrointestinal distress [3]. Clinical protocols often monitor patients with this genetic profile closely to prevent severe adverse reactions [4]. Understanding this status allows healthcare providers to anticipate clearance patterns and adjust supportive care accordingly.
Slower clearance of methotrexate; Increased risk of chemotherapy-induced mouth sores; Altered blood metabolite levels.
Variants with shared genetic architecture
rs4149056 · SLCO1B1 Gene · Chr 12 · Statin Side Effect Risk
rs11045819 · SLCO1B1 Gene · Chr 12 · Statin Response
rs4149081 · SLCO1B1 Gene · Chr 12 · Statin Response
A genome-wide assessment of variability in human serum metabolism.
Genome-wide study of methotrexate clearance replicates SLCO1B1.
PACSIN2 polymorphism influences TPMT activity and mercaptopurine-related gastrointestinal toxicity.
Content updated August 2026.
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