The genetics that changes what anyone does about breast cancer is BRCA1 and BRCA2 — rare, strong, and tested for properly. The variants here are the other kind: common, weak, and useful for understanding the disease rather than for deciding anything.
Breast cancer is where the difference between two kinds of genetics matters more than anywhere else on this site, so this page begins there.
Rare, strong variants in BRCA1 and BRCA2 — and in PALB2, TP53 and a few others — raise lifetime risk substantially. They are found by clinical genetic testing, offered on the basis of family history and other criteria, and a positive result changes real decisions: screening from an earlier age, MRI as well as mammography, discussion of risk-reducing surgery and medication.
Common variants — the kind on this page — shift risk by a few per cent each. Hundreds are known. They explain part of why the disease clusters in families without a strong single-gene cause, and they are not used to decide anything for an individual.
If a family history is worrying you, the useful next step is a conversation about clinical testing, not a consumer chip. And a consumer report that says "no BRCA variants found" usually means a handful of specific variants were checked, not the genes read.
Note what those numbers say about effect size. It took tens of thousands of people to establish these associations, which is what it takes when each variant moves risk a little.
The first study was done in a genetically isolated population, which makes discovery easier and transfer harder. Frequencies and effects estimated in one ancestry do not carry unchanged to another, and most breast cancer genetics has been done in people of European descent. That is a limitation of the evidence, not a statement about anyone.
The things that work are unglamorous and known: attend screening when it is offered, know your family history and tell your doctor about it, and get a new lump, skin or nipple change, or one-sided breast pain looked at promptly. None of that is altered by the positions on this page.
rs2180341 (6q22.33). Identified in a three-phase GWAS in Ashkenazi Jewish women: phase 1 compared 150,080 SNPs in 249 high-risk BRCA1/2 mutation-negative familial cases against 299 cancer-free controls; phase 2 genotyped 343 SNPs from the 123 most significant regions in 950 consecutive cases and 979 age-matched controls; phase 3 replicated in 243 cases and 187 controls. The same study confirmed the FGFR2 region (P = 1.5 x 10-5, OR 1.26, 95% CI 1.13-1.40 at rs1078806 across all phases). The design — familial cases with the high-penetrance genes excluded — is what makes this locus a statement about polygenic background rather than about monogenic risk.
rs4973768 (near SLC4A7). Recorded against a 2011 genome-wide analysis that compared 296,114 tagging SNPs in 1,694 breast cancer cases — 92% with two primary cancers or at least two affected first-degree relatives — against 2,365 controls, with validation in three independent series totalling 11,880 cases and 12,487 controls. That study's own novel finding was 9q31.2 (rs865686, OR 0.89, 95% CI 0.85-0.92, P = 1.75 x 10-10); rs4973768 is among the established loci carried in the same analysis.
Relation to high-penetrance testing. Pathogenic variants in BRCA1, BRCA2, PALB2, TP53, CDH1, STK11 and PTEN carry substantially elevated lifetime risk and drive management: earlier and more intensive surveillance, consideration of risk-reducing mastectomy or salpingo-oophorectomy, and eligibility for PARP inhibitors in the treatment setting. Common variants such as those recorded here are not diagnostic, are not a substitute for clinical testing, and do not modify eligibility for any of it. Polygenic risk scores combining hundreds of common variants have been studied for risk stratification and are not standard of care in most systems.
What a 23andMe/AncestryDNA export or raw VCF can and can't tell you about Breast Cancer 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 |
|---|---|---|
| CYP2D6 | Tamoxifen | A genuine finding that has never settled into a clear instruction, and the page says so rather than choosing a side. Tamoxifen is a prodrug: CYP2D6 turns it into endoxifen, which does most of the antioestrogen work. Patients with reduced-function CYP2D6 genotypes, and patients taking strong CYP2D6 inhibitors, have lower endoxifen levels and — in the guideline's own careful wording — a higher risk of recurrence "in some studies". Trials have disagreed, which is why CPIC issues recommendations while the underlying evidence stays contested. The practical part that is not contested: some antidepressants are strong CYP2D6 inhibitors, and a prescriber choosing one for a woman on tamoxifen will take that into account. Do not stop tamoxifen over this. (CPIC Guideline for CYP2D6 and Tamoxifen Therapy, Clinical Pharmacology & Therapeutics (PMID 29385237)) |
No, and the difference is the most important thing on this page. BRCA1 and BRCA2 testing looks for rare variants that raise risk substantially and that change screening and prevention decisions. These are common variants that shift risk slightly and change nothing clinically.
Not necessarily. Most consumer tests check a small number of specific variants rather than reading the genes, so a negative result does not rule out a pathogenic variant. If your family history worries you, that is a conversation for a clinician.
Because a genetically isolated population makes discovery easier — fewer confounding differences between cases and controls. The trade-off is that frequencies and effects found in one ancestry do not transfer unchanged to another.
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