The enzyme where the broken copy is the common one. Most people of European ancestry carry two of them, which is why the standard dose of one transplant medicine is the dose that suits a non-working enzyme — and why the people whose enzyme works are the ones who get too little.
CYP3A5 is one of a pair of closely related enzymes in the liver and gut. Its partner, CYP3A4, does the bulk of the work on an enormous share of prescribed medicines. CYP3A5 overlaps with it, and for most drugs the difference it makes is swallowed by the larger enzyme beside it.
For one drug it is not swallowed at all, and that drug is tacrolimus — the medicine that stops a transplanted kidney, liver or heart from being rejected.
Almost every page on this site follows the same shape: there is a reference version of a gene that works, and a variant that changes something. CYP3A5 is the other way round.
The *3 allele introduces a splice site where none belongs, and the protein made from it is cut short and does nothing. Roughly five in six people of European ancestry carry two copies of it. The same is true of a large majority of people of East Asian ancestry. In much of sub-Saharan Africa the opposite holds, and most people carry at least one working copy.
So the enzyme that "everybody has" is, in Europe, an enzyme most people do not have. The people described as expressers — the ones whose CYP3A5 is intact — are the exception here and the rule elsewhere.
Tacrolimus has a narrow window. Too little and the transplanted organ is rejected; too much and the drug damages the kidney it was meant to protect. Transplant teams therefore measure the blood level directly and adjust.
An expresser clears tacrolimus faster, and on a standard starting dose reaches a blood level well below target. CPIC's guidance is to start an expresser at about 1.5 to 2 times the standard dose rather than find out over several days of low readings.
The important detail is what the genotype does not change: the target concentration is the same for everybody. Genotype changes how much drug it takes to get there, not where "there" is.
Standard starting doses were set in populations where non-expressers are the overwhelming majority. Expressers are more common in people of African ancestry, and they are the people that standard dose under-treats — in the first days after a transplant, when the consequence of too little drug is rejection.
This is one of the clearest examples in pharmacogenomics of a dose that is not neutral. It was calibrated on one group and applied to everybody, and the group it fits least well is the one where the working enzyme is normal.
What is actually measured. A transplant team measures tacrolimus trough concentrations in blood, frequently at first. Genotype does not replace that and is not intended to: it informs the starting dose, after which the measurement takes over.
What "expresser" means. One functional copy is enough. *1/*1 and *1/*3 are both expressers; only *3/*3 is a non-expresser. This is why a single working copy carries the same recommendation as two.
Other CYP3A5 alleles. *6 and *7 are additional non-functional alleles that occur mainly in people of African ancestry. A genotyping panel that tests only *3 will call some people expressers who are not, which matters most in exactly the population where expressers are common. A test that covers *3 alone is not the same test everywhere.
Beyond tacrolimus. CYP3A5 handles a share of many other drugs, but for almost all of them CYP3A4 covers the difference and no guideline recommends acting on the genotype. Tacrolimus is the exception, not the pattern.
What a 23andMe/AncestryDNA export or raw VCF can and can't tell you about CYP3A5 Metaboliser Status comes down to these specific, well-studied positions — not a diagnosis.
Research-derived gene–drug associations only — not a prescription, dosing guide, or medical advice. Always follow your prescriber's guidance.
| Gene | Drug | What the research shows |
|---|---|---|
| CYP3A5 | Tacrolimus | Tacrolimus prevents rejection of a transplanted organ and has a narrow therapeutic window. CYP3A5 expressers — anybody with at least one functional copy — clear it faster and reach a lower blood level on the same dose. CPIC recommends starting an expresser at about 1.5 to 2 times the standard dose, with the usual therapeutic drug monitoring continuing unchanged, rather than arriving at the right dose through several days of subtherapeutic readings. The target concentration itself does not change. Because functional copies are much more common in people of African ancestry, a single standard starting dose under-treats that group in the days when rejection is the risk. Dosing here is a transplant team’s decision, measured directly in blood, and is not something to act on from a raw data file. (CPIC Guideline for CYP3A5 Genotype and Tacrolimus Dosing (Clin Pharmacol Ther 2015, PMID 25801146).) |
No. It is the most common result in Europe and East Asia, and it is the genotype the standard tacrolimus dose was built around. Nothing about ordinary life is affected by it.
Almost certainly not. Tacrolimus is the one medicine with a guideline attached to this gene. CYP3A5 contributes to the handling of many other drugs, but CYP3A4 beside it covers the difference and no guideline recommends acting on the genotype for them.
No, and this is the part most easily misread. The target is the same for everybody. What differs is the dose needed to reach it. The team measures the level either way.
It depends on ancestry, which is an uncomfortable answer but the accurate one. *6 and *7 are additional non-functional alleles found mainly in people of African ancestry. A test covering only *3 can call somebody an expresser who is not — in precisely the population where the question comes up most.
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