CYP2C9 rs11559021: Understanding the CYP2C9*4 Variant
The rs11559021 variant, commonly identified as the CYP2C9*4 allele, is a genetic change in the CYP2C9 gene that results in a missense mutation (p.Ile359Thr). This variant is associated with reduced enzymatic activity, which may influence how the body processes specific medications like warfarin and phenytoin.
What each genotype means
Typical enzyme activity
This genotype represents the most common form of the CYP2C9 gene, often referred to as the *1 allele. Research suggests this version of the enzyme functions at a standard rate for metabolizing medications like warfarin and phenytoin. If you are prescribed these medications, your clinician will determine your dosage based on standard clinical guidelines.
This is the most common genotype found in the vast majority of the global population.
Reduced enzyme activity
This genotype includes one copy of the rare *4 variant, which has been associated with reduced clearance of certain drugs such as warfarin and phenytoin. Because this variant is extremely rare, clinical evidence regarding its specific impact on drug metabolism is limited. Please discuss your medication management and any potential dosing adjustments with your healthcare provider or pharmacist.
This genotype is extremely rare globally, with occurrences occasionally detected in South Asian cohorts.
Significantly reduced enzyme activity
This genotype indicates the presence of two copies of the *4 variant, which may lead to a more pronounced reduction in the metabolism of substrates like warfarin and phenytoin compared to those with one copy. Due to the extreme rarity of this genotype, scientific data is limited and clinical implications remain uncertain. Always consult with your clinician or pharmacist regarding your specific medication needs.
This genotype is exceptionally rare and is not typically observed in most global population studies.
What is rs11559021?
The rs11559021 variant is a single nucleotide polymorphism (SNP) located within the CYP2C9 gene. In the scientific literature, this specific genetic change is categorized as the CYP2C9*4 allele. It involves a missense mutation where the amino acid isoleucine is replaced by threonine at position 359 of the protein sequence (p.Ile359Thr). This change occurs in a critical region of the enzyme, which can alter its structural integrity and functional capacity. Because the CYP2C9 enzyme is responsible for breaking down a wide variety of drugs, variations like rs11559021 are of significant interest in the field of pharmacogenomics—the study of how genes affect an individual's response to medications. Researchers track these variants to better understand why people may experience different drug effects or side effects when taking the same dosage of a prescribed treatment.
The Role of the CYP2C9 Gene
The CYP2C9 gene provides instructions for making an enzyme that belongs to the cytochrome P450 family. These enzymes are primarily found in the liver and are essential for the metabolism of many substances, including toxins and medications. CYP2C9 is particularly important because it metabolizes several drugs with a narrow therapeutic index, meaning that even small changes in the amount of the drug in the blood can lead to significant differences in efficacy or safety. Common substrates for this enzyme include the anticoagulant warfarin, the anti-seizure medication phenytoin, and various nonsteroidal anti-inflammatory drugs (NSAIDs). When the CYP2C9 enzyme functions normally, it clears these drugs from the body at a predictable rate. However, when genetic variants like rs11559021 reduce the enzyme's activity, the drug may remain in the bloodstream for longer, potentially increasing the risk of adverse reactions.
Research and Clinical Evidence
Research into rs11559021 (CYP2C9*4) suggests that it leads to reduced clearance of medications such as warfarin and phenytoin. While the evidence for this specific variant is considered limited compared to more common variants like *2 or *3, studies indicate that individuals carrying this allele may have decreased metabolic capacity. In clinical settings, reduced metabolism of warfarin can lead to higher drug concentrations, which may increase the risk of over-anticoagulation and bleeding events. Similarly, for phenytoin, reduced metabolism can lead to toxicity if standard doses are used. It is important to note that while genetic associations are well-documented for the CYP2C9 gene as a whole, the clinical impact of any single variant can be influenced by other genetic factors, drug-drug interactions, and individual health status. Consequently, clinicians often use a combination of clinical data and genetic information to guide dosing decisions for these sensitive medications.
Population Frequency
The rs11559021 variant is considered extremely rare in global populations, with an overall frequency of less than 0.001. While it is largely absent in many ethnic groups, it has been occasionally detected in specific South Asian cohorts. Because the variant is so uncommon, it is not typically included in standard, routine pharmacogenetic screening panels, which usually focus on the more prevalent *2 and *3 alleles. The rarity of this variant makes it challenging for researchers to conduct large-scale clinical trials to definitively quantify its impact on drug response. As genomic sequencing becomes more accessible and widespread, our understanding of such rare variants continues to evolve, helping to build a more comprehensive picture of human genetic diversity and its influence on drug metabolism across different ancestral backgrounds.
What This Means for You
If you have information about your CYP2C9 genotype, it is essential to interpret it within the context of your overall health and current medications. Having a variant like rs11559021 does not mean you will necessarily experience a negative reaction to a drug, nor does it serve as a medical diagnosis. Pharmacogenomic information is a tool that clinicians use to help personalize treatment plans, but it is only one piece of the puzzle. You should never change your medication dosage or stop taking a prescribed drug based on genetic test results without first consulting your doctor or pharmacist. These professionals are trained to integrate your genetic profile with other clinical factors—such as your age, weight, liver function, and other medications you may be taking—to ensure your treatment is both safe and effective.
How common is this variant?
The rs11559021 variant is extremely rare, with a global frequency of less than 0.001, and is primarily identified in specific South Asian populations.
Frequently asked questions
What is the CYP2C9*4 variant?
The CYP2C9*4 variant, identified by the SNP rs11559021, is a genetic change that results in a missense mutation in the CYP2C9 enzyme. This mutation is known to reduce the enzyme's ability to metabolize certain drugs.
Does rs11559021 affect warfarin dosing?
Yes, research suggests that variants in the CYP2C9 gene, including those that reduce enzyme activity, can lead to slower clearance of warfarin. This may require a clinician to adjust the starting dose to prevent bleeding risks.
Should I get tested for CYP2C9 variants?
Routine testing for CYP2C9 is not standard for everyone. If you are prescribed medications like warfarin or phenytoin, your doctor may discuss whether pharmacogenetic testing is appropriate for your specific clinical situation.
Is rs11559021 common?
No, rs11559021 is considered extremely rare in the general population. It is found at very low frequencies, primarily in some South Asian cohorts.
Can I change my medication based on this result?
No. You should never change your medication or dosage based on genetic test results without consulting your healthcare provider. Only a clinician can safely interpret these results in the context of your medical history.
Sources & further reading
Educational information only, last refreshed 9/19/2026. Not medical advice — these associations describe population statistics, not individual predictions.
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Rare variant in the CYP2C9 gene potentially impacting enzyme activity and drug metabolism; clinical significance requires further validation.
Coding variant conferring the CYP2C9*3 allele (often mislabeled as *4 in older literature) with decreased enzymatic activity affecting clearance of drugs such as warfarin and phenytoin.
Missense mutation defining the CYP2C9*5 allele that dramatically lowers metabolic activity towards oral anticoagulants and anti-epileptic medications.
A rare variant in the CYP2C9 gene that may impact the metabolism of various clinical drugs.
The CYP2C9*2 variant (p.Arg144Cys) results in reduced enzymatic activity, significantly impacting the metabolism and required dosing of substrates such as warfarin and irbesartan.
Defining the CYP2C9*3 (p.Ile359Leu) allele, this variant markedly decreases enzymatic activity, requiring reduced dosages for warfarin and phenytoin to prevent toxicity.