CYP2C19 rs4244231: Loss-of-Function and Drug Metabolism
The single nucleotide polymorphism rs4244231 is the defining variant of the CYP2C19*2 allele, located in the CYP2C19 gene on chromosome 10. This transition creates an aberrant splice site that eliminates normal enzyme activity, resulting in a loss-of-function phenotype. Carrying this variant significantly alters how the body activates or breaks down medications, including the antiplatelet prodrug clopidogrel and certain proton pump inhibitors.
What each genotype means
Normal enzyme activity
This genotype is typically associated with normal CYP2C19 enzyme function, often referred to as the *1/*1 diplotype. Individuals with this profile generally metabolize medications like clopidogrel at expected rates, though you should always discuss your specific medication needs and dosing with your clinician or pharmacist.
This is the most common genotype globally, found in the majority of individuals across most ancestral populations.
Reduced enzyme activity
This genotype indicates you carry one copy of the loss-of-function *2 allele, which can lead to reduced metabolism of certain drugs such as clopidogrel or specific proton pump inhibitors. Because your enzyme activity may be lower than average, please consult your healthcare provider or pharmacist to determine if this impacts your current or future medication therapy.
Carried by roughly 1 in 3 to 1 in 4 people in many populations, with higher prevalence observed in East Asian groups.
Poor enzyme activity
This genotype indicates you carry two copies of the loss-of-function *2 allele, which is associated with significantly impaired metabolism of medications processed by the CYP2C19 enzyme. If you are prescribed drugs like clopidogrel, this may affect how your body processes the medication; please discuss this result with your clinician or pharmacist to ensure your treatment plan is appropriate for your genetic profile.
This genotype is less common than the heterozygous form, occurring in approximately 2-5% of many populations, though frequencies vary significantly by ancestry.
Molecular Profile and Genetic Context
The variant rs4244231 (historically cataloged in some literature alongside rs4244285) represents a single base substitution (c.681G>A) located within exon 5 of the CYP2C19 gene on chromosome 10q23.33. The CYP2C19 gene encodes cytochrome P450 2C19, an essential member of the hepatic cytochrome P450 superfamily involved in the Phase I oxidative metabolism of numerous xenobiotics and endogenous compounds. The A allele of rs4244231 alters the consensus mRNA donor splice site, leading to alternative, aberrant splicing. This molecular defect introduces a frameshift and premature termination codon, resulting in a truncated, completely non-functional protein product. Because of this complete disruption in enzyme synthesis, the allele defined by this variant—known as CYP2C19*2—is recognized worldwide by pharmacogenomic bodies as an archetype no-function (loss-of-function) allele.
Biological Role and Metabolic Pathways
Under normal physiological conditions, the CYP2C19 enzyme facilitates the biotransformation of approximately 10% of commonly prescribed therapeutic agents. These substrates include cardiovascular therapies such as the thienopyridine antiplatelet agent clopidogrel, selective serotonin reuptake inhibitors like citalopram and escitalopram, tricyclic antidepressants, and proton pump inhibitors (PPIs) including omeprazole and pantoprazole. Clopidogrel is an inactive prodrug that requires a two-step hepatic bioactivation process to form its active thiol metabolite, which subsequently inhibits platelet aggregation by binding to the P2Y12 platelet receptor. CYP2C19 plays an indispensable role in both oxidative activation steps. When enzyme activity is deficient, biotransformation to the active metabolite is drastically decreased, resulting in reduced antiplatelet efficacy. For other medications that are cleared rather than activated by CYP2C19, such as PPIs, diminished enzyme activity leads to elevated drug concentrations.
Pharmacogenomic Evidence and Clinical Associations
Large clinical trials, observational cohorts, and pharmacokinetic investigations have consistently characterized the functional impact of CYP2C19*2 on drug response. Consortia like the Clinical Pharmacogenetics Implementation Consortium (CPIC) and professional bodies like the Association for Molecular Pathology (AMP) classify CYP2C19*2 as a foundational Tier 1 allele. In acute coronary syndrome (ACS) patients undergoing percutaneous coronary intervention (PCI), carriers of loss-of-function alleles receiving standard-dose clopidogrel display significantly higher rates of on-treatment platelet reactivity and heightened risks of major adverse cardiovascular events (MACE), including stent thrombosis and recurrent myocardial infarction. For proton pump inhibitors, reduced metabolism translates to prolonged acid suppression, which can improve Helicobacter pylori eradication rates. However, non-genetic factors—including age, kidney function, and co-administration of enzyme inhibitors (phenoconversion)—also heavily influence drug response.
Population Distribution and Ancestral Patterns
The frequency of the rs4244231 loss-of-function variant varies significantly among different global ancestries. In individuals of European, African, and American continental ancestries, the *2 minor allele frequency typically ranges between 12% and 18%. By contrast, the variant is substantially more prevalent in Asian populations, where allele frequencies generally span 25% to 35%, and reaches even higher rates—up to 60% or more—in specific Oceanian groups such as native populations of Vanuatu and Papua New Guinea. Consequently, poor metabolizer phenotypes resulting from homozygosity (*2/*2) or compound heterozygosity with other non-functional alleles (such as *3) occur in roughly 2% to 4% of Caucasian and African individuals, compared to approximately 14% or more of East Asian individuals.
Interpreting Results and Clinical Perspective
Knowledge of one's rs4244231 genotype provides educational insights into drug metabolism capacity, but it cannot be treated as a direct medical prescription or independent diagnostic finding. Clinical pharmacogenomic interpretation always integrates full diplotype analysis (evaluating both alleles, such as *1/*2, *2/*2, or *2/*17) rather than evaluating a single SNP in isolation. Furthermore, environmental variables, overall health status, liver function, and drug-drug interactions can alter effective enzyme behavior. Individuals discovering they carry one or two copies of this loss-of-function variant should never alter, start, or discontinue any prescribed therapy—such as clopidogrel, PPIs, or antidepressants—on their own. Any treatment modifications or dosing considerations must be evaluated collaboratively with a treating physician or clinical pharmacologist.
How common is this variant?
The minor allele frequency for rs4244231 (*2) is approximately 12–18% in European, African, and American ancestries, rising to 25–35% in East Asian populations and up to 60% in certain Oceanian groups.
Frequently asked questions
What does being a CYP2C19 poor metabolizer mean for my heart medication?
If you carry two non-functional variants like the rs4244231 A allele, your liver produces little to no active CYP2C19 enzyme. Because clopidogrel relies on this enzyme to be converted into its active blood-thinning form, poor metabolizers may experience reduced protection against blood clots. Your cardiologist or pharmacist can discuss alternative medications, such as prasugrel or ticagrelor, which do not rely as heavily on CYP2C19.
Does rs4244231 affect medications other than blood thinners?
Yes, CYP2C19 is responsible for processing many different pharmaceutical classes. For medications that are broken down and cleared by CYP2C19—such as certain proton pump inhibitors (omeprazole) and antidepressants (citalopram)—carrying this variant can cause the drug to clear more slowly, potentially leading to higher active levels in your bloodstream.
Should I stop taking clopidogrel if I discover I carry the A allele?
No, you should never discontinue or change the dose of any prescription medication based solely on a genetic test result. Abruptly stopping antiplatelet therapy can dramatically increase the immediate risk of blood clots, heart attack, or stroke. Always consult your treating physician or cardiologist first to review your complete medical profile and determine if therapy adjustments are warranted.
Can a person with the rs4244231 variant still respond normally to medications?
Yes, because drug response is shaped by multiple genetic and clinical factors. Other enzymes can sometimes partially compensate, and co-prescribed medications or diet can alter enzyme activity through phenoconversion. A genetic variant indicates statistical likelihoods in metabolic rate rather than an absolute clinical outcome.
Sources & further reading
Educational information only, last refreshed 9/7/2026. Not medical advice — these associations describe population statistics, not individual predictions.
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The CYP2C19*2 loss-of-function splicing defect results in impaired biotransformation of prodrugs including clopidogrel and altered clearance of proton pump inhibitors and select antidepressants.
Defines the CYP2C19*17 increased function allele, conferring rapid or ultrarapid metabolism for CPIC-actionable drugs including clopidogrel, PPIs, and SSRIs.
Variant in the CYP2C19 locus associated with altered metabolism of clopidogrel and proton-pump inhibitors.
Defines the CYP2C19*8 loss-of-function variant (c.358T>C, p.Trp120Arg) which results in no enzymatic activity and impaired metabolism of CYP2C19 substrates.
Defines the non-functional CYP2C19*3 null allele resulting from a premature stop codon, leading to poor metabolism of drugs such as clopidogrel, escitalopram, and proton pump inhibitors.
Defines the CYP2C19*4 allele caused by an ATG start codon alteration that abolishes translation, resulting in reduced or absent drug metabolism capacity.