What you carry at this position in your DNA and what research says about it.
Chemotherapy Toxicity Risk: 8/10 evidence, 6 publications.
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Sample genotype (not yours): A/A
Normal Chemotherapy Drug Clearance
Standard clearance of fluoropyrimidine-based chemotherapy drugs is expected. No copies of the non-functional DPYD*13 variant were detected.
What this genotype means: Normal breakdown of fluorouracil; Standard chemotherapy dosing is typically appropriate.
Sample genotype (not yours): A/C
High Chemotherapy Toxicity Risk
One copy of the non-functional DPYD*13 variant is present, significantly reducing the body's ability to clear certain chemotherapy drugs. Standard doses of fluorouracil or capecitabine may lead to severe, life-threatening side effects.
What this genotype means: Significantly reduced drug clearance; Increased risk of severe neutropenia and diarrhea; Severely reduced tolerance; alternative therapy usually needed.
Sample genotype (not yours): C/C
Extreme Chemotherapy Toxicity Risk
Two copies of the non-functional DPYD*13 variant are present, causing complete dihydropyrimidine dehydrogenase deficiency. Standard fluoropyrimidine chemotherapies are highly dangerous and must be avoided.
What this genotype means: Complete lack of drug clearance; Extremely high risk of fatal chemotherapy toxicity; Alternative chemotherapy agents are strongly recommended.
Gene function and biological role
Gene Function: Dihydropyrimidine dehydrogenase, encoded by the DPYD gene, is the rate-limiting enzyme responsible for breaking down pyrimidines, including common chemotherapy drugs [1]. Under normal conditions, this enzyme rapidly catabolizes approximately 85% of administered fluorouracil in the liver, preventing dangerous drug accumulation [1]. When the DPYD gene carries certain variants, enzyme activity is significantly reduced or completely absent [1]. This deficiency leads to a prolonged drug half-life and a dramatic increase in the risk of severe, potentially life-threatening toxicities during chemotherapy [2].
Variant Impact: Replacing the amino acid isoleucine with serine at position 560 of the dihydropyrimidine dehydrogenase enzyme creates the DPYD*13 allele [1]. This specific missense change, also designated as c.1679T>G, results in a completely non-functional enzyme [1]. Because the variant allele has no function, individuals carrying even one copy have a significantly reduced capacity to clear fluoropyrimidine chemotherapies [1]. Clinical studies show that this variant is one of the strongest genetic predictors of severe early-onset chemotherapy side effects [1].
Key Insight: Even a single copy of this rare variant can reduce the body's ability to clear common chemotherapy drugs, turning standard doses into toxic overdoses.
Location: DPYD. Chromosome 1 · 1p21.3 · 97.5 Mb
Estimated prevalence by population
Global 99.9%; European 99.9%; African > 99.9%; East Asian 100.0%; South Asian 100.0%; Americas 100.0%; Ashkenazi 100.0%; Finnish > 99.9%; Middle Eastern 100.0%; Amish 100.0%.
Estimated AA genotype frequency.
Global 0.1%; European 0.1%; African < 0.1%; East Asian 0.0%; South Asian 0.0%; Americas 0.0%; Ashkenazi 0.0%; Finnish < 0.1%; Middle Eastern 0.0%; Amish 0.0%.
Estimated AC genotype frequency.
Global < 0.1%; European < 0.1%; African < 0.1%; East Asian 0.0%; South Asian 0.0%; Americas 0.0%; Ashkenazi 0.0%; Finnish < 0.1%; Middle Eastern 0.0%; Amish 0.0%.
Estimated CC genotype frequency.
Showing effects for A/A genotype
Pharmacogenomics
Chemotherapy Response
Fluorouracil Toxicity. Evidence: 8/10. Source: PMID 26603945. Typical Risk.
Capecitabine Toxicity. Evidence: 8/10. Source: PMID 26603945. Typical Risk.
Tegafur Toxicity. Evidence: 8/10. Source: PMID 26603945. Typical Risk.
Showing effects for A/C genotype
Fluorouracil Toxicity. Evidence: 8/10. Source: PMID 26603945. High Toxicity Risk.
Capecitabine Toxicity. Evidence: 8/10. Source: PMID 26603945. High Toxicity Risk.
Tegafur Toxicity. Evidence: 8/10. Source: PMID 26603945. High Toxicity Risk.
Showing effects for C/C genotype
Fluorouracil Toxicity. Evidence: 8/10. Source: PMID 26603945. Extreme Toxicity Risk.
Capecitabine Toxicity. Evidence: 8/10. Source: PMID 26603945. Extreme Toxicity Risk.
Tegafur Toxicity. Evidence: 8/10. Source: PMID 26603945. Extreme Toxicity Risk.
What this genotype means for the people who carry it
Sample genotype: AA (Homozygous)
Normal dihydropyrimidine dehydrogenase enzyme activity allows for the typical breakdown and clearance of fluoropyrimidine drugs like fluorouracil and capecitabine [1]. Clinical guidelines indicate that individuals with this genotype can generally receive standard starting doses of these chemotherapies without an elevated genetic risk of severe toxicity [1]. However, non-genetic factors such as age, kidney function, and drug interactions still play a role in overall drug tolerance. Standard clinical monitoring remains necessary during treatment [2]. A normal genetic profile of this type represents the most common profile globally, occurring in over 99% of the population.
Normal breakdown of fluorouracil; Standard chemotherapy dosing is typically appropriate.
Sample genotype: AC (Heterozygous)
Presence of one copy of the non-functional DPYD*13 allele results in partial dihydropyrimidine dehydrogenase deficiency [1]. The body's capacity to break down fluoropyrimidine chemotherapies is reduced by approximately half, causing these drugs to accumulate to dangerous levels [1]. These toxicities can include severe diarrhea, painful hand-foot syndrome, extreme white blood cell depletion, and life-threatening infections [2]. Prospective genetic testing is increasingly utilized in clinical settings to identify carriers before chemotherapy begins [2]. Alternative treatments or carefully personalized dosing strategies are typically discussed by oncology teams in these cases.
Significantly reduced drug clearance; Increased risk of severe neutropenia and diarrhea; Severely reduced tolerance; alternative therapy usually needed.
Sample genotype: CC (Homozygous)
Inheriting two copies of the non-functional DPYD*13 allele leads to a complete lack of dihydropyrimidine dehydrogenase enzyme activity [1]. Without this vital enzyme, the body is virtually unable to break down and clear fluoropyrimidine chemotherapies such as fluorouracil, capecitabine, and tegafur [1]. Administering standard doses of these medications to individuals with complete deficiency can result in profound, life-threatening systemic toxicity [2]. Severe complications include extreme bone marrow suppression, widespread mucosal ulceration, severe gastrointestinal distress, and multi-organ failure [2]. Clinical guidelines strongly recommend avoiding these specific chemotherapy agents entirely, opting instead for alternative therapeutic options [1]. A homozygous profile of this type is exceptionally rare but represents a critical clinical finding that requires immediate medical attention prior to any cancer treatment.
Complete lack of drug clearance; Extremely high risk of fatal chemotherapy toxicity; Alternative chemotherapy agents are strongly recommended.
Variants with shared genetic architecture
rs56038477 · DPYD Gene · Chr 1 · Chemotherapy Toxicity Risk
rs75017182 · DPYD Gene · Chr 1 · Chemotherapy Drug Toxicity
rs17376848 · DPYD Gene · Chr 1 · Chemotherapy Toxicity Risk
Content updated August 2026.
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