ABCB1 rs1128503: What Your Genotype Means
The rs1128503 variant is a common genetic change located within the ABCB1 gene, which provides instructions for making a protein that acts as a cellular pump. Research explores whether this variant influences how the body absorbs, distributes, or eliminates certain medications, though findings remain inconsistent across different populations and drug types.
What each genotype means
Standard transporter activity
This genotype represents the common form of the ABCB1 gene. Some research suggests that individuals with this genotype may experience different rates of side effects, such as nausea or vomiting, when taking certain chemotherapy medications compared to those with the T allele. Please discuss any concerns regarding medication side effects or dosing with your clinician or pharmacist.
This genotype is common, occurring in roughly 25% of many global populations.
Intermediate transporter activity
This genotype indicates you carry one copy of each common variant of the ABCB1 gene. Studies have explored whether this variation influences how your body processes certain drugs, with some evidence linking it to differences in chemotherapy-related side effects. Because the evidence is limited and mixed, please consult your healthcare provider regarding your specific medication management.
This is a very common genotype, found in approximately 50% of many global populations.
Variant transporter activity
This genotype represents the variant form of the ABCB1 gene, which is a synonymous change that does not alter the protein structure. Some studies have observed that individuals with this genotype may have different risks for certain side effects or drug responses compared to those with the C allele. As research findings are inconsistent across different drugs and populations, please discuss your treatment plan with your doctor or pharmacist.
This genotype is common, occurring in roughly 25% of many global populations.
Understanding the rs1128503 Variant
The rs1128503 variant is a single-nucleotide polymorphism (SNP) found in the ABCB1 gene, also historically known as the MDR1 (multidrug resistance 1) gene. In genetic terms, this variant is a synonymous polymorphism, meaning that while it changes the DNA sequence, it does not alter the primary amino acid sequence of the resulting protein. Despite this, researchers have long investigated whether such variants might influence the stability of the messenger RNA or the efficiency with which the protein is produced and folded. Located within the coding region of the gene, rs1128503 is one of the most frequently studied markers in pharmacogenomics. Because the ABCB1 gene is highly polymorphic, scientists often study this variant alongside others, such as rs1045642 and rs2032582, to understand how inherited genetic patterns might contribute to individual differences in drug response.
The Role of the ABCB1 Gene
The ABCB1 gene encodes a protein called P-glycoprotein (P-gp). This protein functions as an ATP-dependent efflux pump, which is embedded in the membranes of various cells throughout the body, including those in the intestines, liver, kidneys, and the blood-brain barrier. Its primary job is to transport a wide variety of substances, including many pharmaceutical drugs, out of cells. By pumping these substances back into the bloodstream or into the gut lumen, P-glycoprotein effectively limits the amount of medication that reaches its target site or enters the central nervous system. Because of this role, variations in the ABCB1 gene are hypothesized to potentially affect the pharmacokinetics—how a drug moves through the body—of many common medications. When P-glycoprotein activity is altered, it may theoretically lead to higher or lower concentrations of a drug in the blood, which could influence both the efficacy and the potential side effects of a treatment.
Research Associations and Evidence Strength
The evidence linking rs1128503 to clinical outcomes is considered limited and often conflicting. Numerous studies have examined its role in cancer treatment, particularly in patients receiving chemotherapy agents like taxanes or temozolomide. Some research suggests that certain genotypes may be associated with better survival or different drug plasma levels, while other studies find no significant correlation. For example, meta-analyses have occasionally reported that carriers of specific alleles might experience different outcomes, but these results are frequently inconsistent across different ethnic groups and tumor types. Furthermore, studies investigating the role of this variant in the metabolism of other drugs, such as statins or anticoagulants, have often failed to find a clear, clinically relevant impact. Because of this heterogeneity in findings, the scientific community generally views the clinical utility of testing for this specific variant as currently unproven for routine medical decision-making.
What This Information Means for You
If you have received information about your rs1128503 genotype, it is important to understand that this variant is not a diagnostic tool. The presence of a specific genotype does not guarantee a particular response to any medication, nor does it predict the development of any disease. Because the scientific evidence is mixed and often depends on the specific drug, the patient's ancestry, and other environmental factors, this genetic information should not be used to make changes to your prescribed treatments. If you are concerned about how a medication is affecting you, or if you are interested in how your genetics might influence your health, the best course of action is to have a conversation with your doctor or pharmacist. They can provide context based on your full medical history and current clinical guidelines, which are far more reliable than individual genetic markers for managing your care.
How common is this variant?
The rs1128503 variant is highly common, with the minor allele frequency typically ranging between 0.40 and 0.50 across most global populations.
Frequently asked questions
Does the rs1128503 variant mean I am resistant to medication?
No, this variant does not mean you are resistant to medication. While some research explores whether it influences how drugs are processed, the evidence is not strong enough to suggest it causes clinical drug resistance.
Should I change my medication based on my ABCB1 genotype?
You should never change your medication based on genetic test results without consulting your healthcare provider. Only a doctor or pharmacist can determine if a change is appropriate based on your overall health.
Is rs1128503 the same as the MDR1 gene?
rs1128503 is a specific genetic variant located within the ABCB1 gene, which is also known as the MDR1 gene. The gene provides the instructions for the protein, while the variant is a single change in the DNA sequence of that gene.
Why do different studies report different results for this variant?
Genetic studies often show different results due to variations in study design, the specific drugs being tested, and the diverse genetic backgrounds of the participants. This inconsistency is why the evidence for this variant is currently considered limited.
Sources & further reading
Educational information only, last refreshed 10/16/2026. Not medical advice — these associations describe population statistics, not individual predictions.
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This variant is studied for its influence on the pharmacokinetics of immunosuppressive drugs like cyclosporine and tacrolimus in transplant patients.
Synonymous variant (c.3435C>T) in ABCB1 linked to altered P-glycoprotein expression and multi-drug transporter pharmacokinetics.
Nonsynonymous variant in ABCB1 altering P-glycoprotein transporter kinetics.
This SNP is linked to variations in the pharmacokinetics of drugs that are substrates for the ABCB1 transporter protein.
A variant associated with potential differences in drug absorption and clinical outcomes in patients undergoing specific pharmacological therapies.
Synonymous exon 26 variant in ABCB1 linked to altered mRNA stability and variable response to multiple cardiovascular medications.