What you carry at this position in your DNA and what research says about it.
Chemotherapy Toxicity Risk: 8/10 evidence, 13 publications.
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Sample genotype (not yours): T/T
Typical Chemotherapy Toxicity Risk
Normal dihydropyrimidine dehydrogenase enzyme activity is expected with this profile.
What this genotype means: Normal drug metabolism; Standard chemotherapy tolerance.
Sample genotype (not yours): C/C
Two copies of the C allele are associated with normal dihydropyrimidine dehydrogenase activity under major guidelines.
Sample genotype (not yours): T/C
One copy of the C allele is generally associated with normal dihydropyrimidine dehydrogenase activity.
Gene function and biological role
Gene Function: The DPYD gene encodes dihydropyrimidine dehydrogenase, a crucial enzyme responsible for breaking down pyrimidines like uracil and thymine [1]. This enzyme is also the primary rate-limiting factor involved in metabolizing fluoropyrimidine chemotherapy drugs, such as 5-fluorouracil and capecitabine [2]. When enzyme activity is reduced or absent, these drugs can accumulate to toxic levels in the body [3]. This accumulation can lead to severe, life-threatening side effects, including severe diarrhea, low white blood cell counts, and nerve damage [3]. Understanding genetic variations in this gene is therefore essential for personalizing chemotherapy dosing and preventing severe adverse drug reactions [4].
Variant Impact: The rs1801159 variant, also known as DPYD*5, is a common genetic change in the DPYD gene that results in the substitution of the amino acid valine for isoleucine at position 543 of the protein [5]. This missense variant is highly prevalent across global populations, with the minor C allele occurring in approximately 18 percent of individuals [3]. Clinical guidelines from the Clinical Pharmacogenetics Implementation Consortium classify both the T and C alleles of this variant as having normal function [2]. However, some smaller clinical studies have historically linked the C allele to reduced enzyme activity and an increased risk of toxicity from fluorouracil-based chemotherapies [5]. Despite these conflicting reports, large-scale cohort studies have not reproduced these findings, leading major expert panels to consider it a benign polymorphism [2].
Key Insight: Although some early studies flagged the DPYD*5 variant as a risk factor for chemotherapy toxicity, large-scale clinical guidelines now classify it as completely normal and safe.
Location: DPYD. Chromosome 1 · 1p21.3 · 97.5 Mb
Estimated prevalence by population
Global 66.3%; European 64.7%; African 70.8%; East Asian 55.5%; South Asian 81.6%; Americas 58.2%; Ashkenazi 64.9%; Finnish 69.4%; Middle Eastern 72.7%; Amish 75.2%.
Estimated TT genotype frequency.
Global 30.1%; European 31.5%; African 26.7%; East Asian 38.0%; South Asian 17.4%; Americas 36.2%; Ashkenazi 31.3%; Finnish 27.8%; Middle Eastern 25.1%; Amish 23.1%.
Estimated TC genotype frequency.
Global 3.6%; European 3.8%; African 2.5%; East Asian 6.5%; South Asian 0.9%; Americas 5.6%; Ashkenazi 3.8%; Finnish 2.8%; Middle Eastern 2.2%; Amish 1.8%.
Estimated CC genotype frequency.
Showing effects for T/T genotype
Pharmacogenomics
Chemotherapy Response
Fluorouracil Toxicity. Evidence: 8/10. Source: PMID 23988873. Typical Risk.
Capecitabine Toxicity. Evidence: 8/10. Source: PMID 32378051. Typical Risk.
Tegafur Response. Evidence: 5/10. Source: PMID 18537153. Typical Response.
Drug Metabolism Enzyme Activity
DPD Enzyme Activity. Evidence: 8/10. Source: PMID 17848752. Normal Activity.
Showing effects for C/C genotype
Showing effects for T/C genotype
What this genotype means for the people who carry it
Sample genotype: TT (Homozygous)
Two copies of the common T allele represent the standard genetic profile for the DPYD gene. The Clinical Pharmacogenetics Implementation Consortium classifies this genotype as having normal enzyme activity [1]. Consequently, individuals with this genotype are expected to metabolize fluoropyrimidine drugs, such as 5-fluorouracil and capecitabine, at a typical rate [1]. This means there is no genetically elevated risk of severe chemotherapy-induced side effects like neutropenia, severe diarrhea, or hand-foot syndrome [2]. While some early, small-scale studies suggested that other variants might alter drug response, large clinical trials have confirmed that this genotype represents the baseline population risk [3].
Normal drug metabolism; Standard chemotherapy tolerance.
Sample genotype: CC (Homozygous)
This genotype consists of two copies of the variant C allele. Although some early pharmacogenomic studies and smaller patient cohorts suggested that the C/C genotype might lead to slower metabolism of fluoropyrimidine drugs and a higher risk of adverse events like nausea or low white blood cell counts, major clinical consensus has not supported this [1] [2]. The Clinical Pharmacogenetics Implementation Consortium classifies both alleles of this variant as having normal function [3]. Therefore, individuals with this genotype are clinically considered normal metabolizers of chemotherapy drugs like 5-fluorouracil and capecitabine [3].
Sample genotype: TC (Heterozygous)
This heterozygous genotype carries one copy of the common T allele and one copy of the variant C allele. According to the Clinical Pharmacogenetics Implementation Consortium, both alleles of this variant are assigned normal function, meaning this genotype is clinically classified as a normal metabolizer [1]. Large-scale clinical cohorts have shown no significant increase in severe toxicities, such as low white blood cell counts or severe diarrhea, during fluorouracil or capecitabine therapy [2]. However, the scientific literature contains some conflicting reports, with a few smaller studies suggesting a slight reduction in enzyme activity or a higher rate of side effects in C allele carriers [3]. Because these findings have not been consistently replicated in larger populations, standard clinical guidelines do not recommend preemptive dose reductions for this genotype [1]. Patients should always discuss their full genetic profile and potential chemotherapy side effects with their oncology team [4].
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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