What you carry at this position in your DNA and what research says about it.
Chemotherapy Toxicity Risk: 8/10 evidence, 19 publications.
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Sample genotype (not yours): C/C
Typical Chemotherapy Toxicity Risk
Normal dihydropyrimidine dehydrogenase enzyme function allows for standard clearance of fluoropyrimidine chemotherapies.
What this genotype means: Normal DPYD enzyme activity; Standard clearance of fluoropyrimidine drugs; Typical risk of chemotherapy side effects.
Sample genotype (not yours): C/T
High Chemotherapy Toxicity Risk
Reduced dihydropyrimidine dehydrogenase enzyme activity significantly increases the risk of severe toxicity from standard doses of fluoropyrimidine chemotherapies. Reduced tolerance to fluoropyrimidine chemotherapy.
What this genotype means: Partial DPYD enzyme deficiency; Impaired clearance of fluoropyrimidine drugs; Significantly increased risk of severe chemotherapy toxicity.
Sample genotype (not yours): T/T
Extremely High Chemotherapy Toxicity Risk
Complete dihydropyrimidine dehydrogenase deficiency leads to an extremely high risk of fatal toxicity if standard doses of fluoropyrimidine chemotherapies are administered.
What this genotype means: Complete DPYD enzyme deficiency; Inability to clear fluoropyrimidine drugs; Extremely high risk of fatal chemotherapy toxicity.
Gene function and biological role
Gene Function: The DPYD gene encodes dihydropyrimidine dehydrogenase, an enzyme responsible for breaking down uracil and thymine [1]. This enzyme also plays a critical role in metabolizing fluoropyrimidine chemotherapy drugs, such as 5-fluorouracil and capecitabine [2]. Under normal conditions, it clears more than 80% of these drugs from the body to prevent toxic buildup [2]. A deficiency in this enzyme prevents proper clearance, allowing active drug metabolites to accumulate to dangerous levels [2]. This accumulation can result in severe, potentially life-threatening chemotherapy-induced toxicity [2].
Variant Impact: A single nucleotide transition (C to T) located at an invariant splice donor site of the DPYD gene defines this genetic change [3]. This specific alteration disrupts normal pre-mRNA splicing, causing the entire exon 14 to be skipped during transcription [4]. The resulting mRNA transcript is truncated, producing a completely non-functional dihydropyrimidine dehydrogenase enzyme [4]. Known clinically as the DPYD*2A allele, this variant is the most common genetic cause of severe dihydropyrimidine dehydrogenase deficiency [5]. Even carrying a single copy of this variant significantly reduces the body's ability to clear fluoropyrimidine chemotherapies safely [6].
Key Insight: A single letter change causes an entire section of the gene to be skipped during protein assembly, completely disabling the enzyme. While rare, this variant is one of the most clinically important genetic markers for predicting severe chemotherapy side effects.
Location: DPYD. Chromosome 1 · 1p21.3 · 97.5 Mb
Estimated prevalence by population
Global 99.1%; European 99.0%; African 99.9%; East Asian 100.0%; South Asian 99.4%; Americas 99.7%; Ashkenazi 98.6%; Finnish 95.2%; Middle Eastern 100.0%; Amish 100.0%.
Estimated CC genotype frequency.
Global 0.9%; European 1.0%; African 0.1%; East Asian 0.0%; South Asian 0.6%; Americas 0.3%; Ashkenazi 1.4%; Finnish 4.7%; Middle Eastern 0.0%; Amish 0.0%.
Estimated CT genotype frequency.
Global < 0.1%; European < 0.1%; African < 0.1%; East Asian 0.0%; South Asian < 0.1%; Americas < 0.1%; Ashkenazi < 0.1%; Finnish 0.1%; Middle Eastern 0.0%; Amish 0.0%.
Estimated TT genotype frequency.
Showing effects for C/C genotype
Pharmacogenomics
Chemotherapy Response
Fluorouracil Toxicity. Evidence: 8/10. Source: PMID 9470816. Typical Risk.
Capecitabine Toxicity. Evidence: 8/10. Source: PMID 26603945. Typical Risk.
Tegafur Toxicity. Evidence: 8/10. Source: PMID 23930673. Typical Risk.
Nutrition & Metabolism
Blood Chemistry & Metabolites
Uracil Levels. Evidence: 6/10. Source: PMID 35347128. Typical.
Showing effects for C/T genotype
Fluorouracil Toxicity. Evidence: 8/10. Source: PMID 9470816. High Risk.
Capecitabine Toxicity. Evidence: 8/10. Source: PMID 26603945. High Risk.
Tegafur Toxicity. Evidence: 8/10. Source: PMID 23930673. High Risk.
Uracil Levels. Evidence: 6/10. Source: PMID 35347128. Elevated.
Showing effects for T/T genotype
Fluorouracil Toxicity. Evidence: 8/10. Source: PMID 9470816. Very High Risk.
Capecitabine Toxicity. Evidence: 8/10. Source: PMID 26603945. Very High Risk.
Tegafur Toxicity. Evidence: 8/10. Source: PMID 23930673. Very High Risk.
Uracil Levels. Evidence: 6/10. Source: PMID 35347128. Highly Elevated.
What this genotype means for the people who carry it
Sample genotype: CC (Homozygous)
Normal dihydropyrimidine dehydrogenase activity is present, meaning the body can efficiently break down and clear fluoropyrimidine chemotherapy drugs [1]. Because the enzyme functions normally, the risk of experiencing severe, life-threatening toxicities from these specific drugs is not elevated [3]. However, other genetic and clinical factors can still influence how a person responds to chemotherapy [4]. This genotype represents the typical status found in the vast majority of the global population [1]. It is considered the baseline reference for drug metabolism studies [2].
Normal DPYD enzyme activity; Standard clearance of fluoropyrimidine drugs; Typical risk of chemotherapy side effects.
Sample genotype: CT (Heterozygous)
Presence of a single copy of the non-functional *2A variant results in partial dihydropyrimidine dehydrogenase deficiency [1]. This reduction in enzyme activity impairs the body's ability to clear fluoropyrimidine chemotherapy drugs like fluorouracil and capecitabine [2]. Standard doses of these medications can lead to rapid drug accumulation, causing severe toxicities such as profound neutropenia, severe diarrhea, and mucositis [3]. This variant is relatively rare but represents one of the most actionable pharmacogenomic markers in oncology [3].
Partial DPYD enzyme deficiency; Impaired clearance of fluoropyrimidine drugs; Significantly increased risk of severe chemotherapy toxicity.
Sample genotype: TT (Homozygous)
Inheriting two copies of the non-functional *2A variant causes complete dihydropyrimidine dehydrogenase deficiency [1]. The body is virtually unable to metabolize fluoropyrimidine chemotherapies, leading to rapid, lethal drug accumulation [2]. Standard doses of fluorouracil or capecitabine can result in catastrophic toxicities, including severe bone marrow suppression, organ failure, and death [3]. This genotype also leads to hereditary thymine-uraciluria, characterized by elevated levels of uracil and thymine in bodily fluids [5]. Although extremely rare in the general population, its clinical impact is profound [1].
Complete DPYD enzyme deficiency; Inability to clear fluoropyrimidine drugs; Extremely high risk of fatal chemotherapy toxicity; Hereditary thymine-uraciluria.
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
Genotype and phenotype in patients with dihydropyrimidine dehydrogenase deficiency.
Content updated September 2026.
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