Tamoxifen Response — CYP2D6 variants that determine whether the most widely prescribed breast cancer endocrine therapy produces its active metabolite or remains an ineffective prodrug.
Whole genome sequencing provides the complete CYP2D6 diplotype needed to implement CPIC-guided tamoxifen prescribing — the information that determines whether a breast cancer patient should receive tamoxifen or an aromatase inhibitor.
Tamoxifen Response — CYP2D6
Tamoxifen is a selective estrogen receptor modulator (SERM) used as adjuvant endocrine therapy for estrogen receptor-positive (ER+) breast cancer — the most common breast cancer subtype. Tamoxifen is a prodrug that requires metabolic activation, primarily by CYP2D6, to produce endoxifen (4-hydroxy-N-desmethyltamoxifen), the active metabolite responsible for the majority of tamoxifen's anti-estrogenic activity. Endoxifen has approximately 100-fold higher anti-estrogenic potency than the parent drug tamoxifen. CYP2D6 metabolizer status directly determines endoxifen plasma concentrations — and therefore tamoxifen's therapeutic efficacy.
CYP2D6 poor metabolizers (PMs), who carry two loss-of-function alleles, achieve endoxifen concentrations substantially below the therapeutic threshold (approximately 5.9 ng/mL). Multiple large studies — including the BIG 1-98 trial, ATAC secondary analyses, and ABCSG 8 trial data — have demonstrated that CYP2D6 PMs on tamoxifen have higher breast cancer recurrence rates compared to extensive metabolizers on tamoxifen or compared to tamoxifen PMs switched to aromatase inhibitors. Intermediate metabolizers (IMs), who carry one loss-of-function allele, achieve intermediate endoxifen levels and have a smaller but measurable increase in recurrence risk.
The CPIC Level A guideline for CYP2D6 and tamoxifen recommends modified prescribing based on metabolizer phenotype: extensive and ultra-rapid metabolizers may use standard tamoxifen dosing; intermediate metabolizers may be considered for higher tamoxifen doses (40 mg) to achieve therapeutic endoxifen levels, or alternative endocrine therapy; and poor metabolizers should receive an alternative endocrine agent — typically an aromatase inhibitor for postmenopausal women, or aromatase inhibitor with ovarian suppression for premenopausal women. This is one of the most impactful pharmacogenomic decisions in oncology, as tamoxifen adjuvant therapy is typically prescribed for 5-10 years.
CYP2D6 genotyping before initiating 5-10 years of adjuvant tamoxifen determines whether the drug will produce therapeutic endoxifen levels — a decision that directly affects breast cancer recurrence risk.
A 5-10 year treatment decision deserves a complete CYP2D6 genotype — not a limited panel result
Tamoxifen adjuvant therapy is prescribed for 5 to 10 years. A CYP2D6 poor metabolizer who remains on tamoxifen for this entire duration receives a decade of subtherapeutic endocrine therapy — significantly increasing breast cancer recurrence risk compared to what an aromatase inhibitor would have provided. Standard point-of-care pharmacogenomics panels test a limited set of CYP2D6 variants and may not reliably detect gene deletions (*5) or duplications that define the complete metabolizer phenotype. A comprehensive CYP2D6 diplotype from whole genome sequencing — including copy number analysis — provides the definitive result that a decade-long treatment decision warrants.
Co-prescribed CYP2D6 inhibitors convert normal metabolizers into functional poor metabolizers
Strong CYP2D6 inhibitors — including paroxetine and fluoxetine, commonly co-prescribed for tamoxifen-associated hot flashes and depression — convert CYP2D6 normal metabolizers into phenotypic poor metabolizers through enzyme inhibition. This drug-drug interaction is well-documented and has led to guidelines against co-prescribing strong CYP2D6 inhibitors with tamoxifen. However, the interaction is most dangerous in CYP2D6 intermediate metabolizers — patients who have marginally sufficient CYP2D6 activity that a co-prescribed inhibitor pushes below the therapeutic threshold. Knowing the baseline CYP2D6 diplotype identifies which patients are at highest risk from this interaction and informs the choice of alternative medications for managing tamoxifen side effects.
Your full DNA (not just a part of it)
Traditional genetic testing looks at narrow sets of genes, missing most parts of your genome. We sequence your full genome — every gene and every region between genes.
Comprehensive insights and specialized reports
Easy to read and with answers you and your doctor can act on. Not a file to interpret — 200+ clinical reports, organized by category.
Your test becomes more valuable every year
Your DNA does not change, but genome science is accelerating. Every month, new variant-disease associations are discovered. We validate these findings and update your reports automatically. Your test becomes more valuable every year.
The results doctors bring to their hardest cases.
Forty years of uncertainty. One test.
A patient had spent decades in the UK healthcare system without a diagnosis. Dante data, accepted by NHS clinical teams at Queen Elizabeth University Hospital Glasgow, identified Noonan Syndrome and a RUNX1 leukemia-associated variant that had gone undetected. After 40 years, they finally had an answer.
A complete read delivers a complete picture.
A patient came to Dante to investigate periodic paralysis. Reading the complete genome identified a concurrent hereditary cardiac finding — Brugada syndrome — that their doctor confirmed with an ECG. The result also explained a family member's unresolved cardiac history. One test. Every answer in it.
Sequenced in 2019. The data worked in 2021.
Jennifer sequenced her genome with Dante two years before her breast cancer diagnosis. When treatment began, Dante's pharmacogenomics data showed her prescribed chemotherapy would cause serious adverse effects. Her doctor selected an alternative — and she started effective treatment from day one.
Every genetic question deserves a complete answer.
Whether you are searching for answers today or protecting your health for tomorrow, a complete read of your entire genome is the only place to start.
It runs in your family. Now you can know if it runs in your genes.
Your genome contains inherited variants associated with medical conditions like cardiac, cancer, and neurological. We read all of them — with the clinical depth to give the result meaning.
Learn more →When traditional lab tests say you're fine. And you know you're not.
Standard diagnostic tests check for a pre-selected set of answers. We sequence your full DNA — including parts that no test was designed to check. If the answer is in your genome, we will help you find it.
Learn more →Your diagnosis may be right. Your treatment plan may be incomplete.
Your genes determine which treatments are most likely to work — and which are not. We give your doctor the tools and insights to inform your treatment plan.
Learn more →You want to know before something forces the question.
Some people don't wait for a diagnosis or a family history to act. Whole genome sequencing gives you the complete genetic picture now — so you and your doctor can make informed decisions before anything becomes urgent.
Learn more →You already took a DNA test. Here's what it couldn't tell you.
Most consumer DNA tests read less than 0.1% of your genome. We read all of it.
Learn more →Clinical-grade results. Chosen by individuals, trusted by doctors for their most complex cases.
Dante Genome Test helped specialists at a UK national acute hospital in the identification of Noonan Syndrome and a rare leukemia-associated genetic variant that had gone undetected. That result changed the medical care of the patient.
Accredited by & published in
Common questions about whole genome sequencing.
What is the difference between whole genome sequencing and a targeted genetic test?
Targeted genetic tests — including standard hereditary cancer panels — read a pre-defined list of known variants in a specific set of genes. They are designed to find what they already know to look for. Whole genome sequencing reads your entire genome: all 6 billion base pairs, every gene, every region between genes. A Mayo Clinic study published in JAMA Oncology found that standard testing guidelines missed more than half of patients with inherited cancer mutations. Genome Test does not have a fixed list.
What will I receive when my results are ready?
Your Dante Genome delivers 200+ physician-ready reports organized by clinical category — hereditary cancer, cardiac conditions, rare diseases, pharmacogenomics, carrier status, and more. Reports are delivered to your secure Genome Manager and are formatted for direct clinical use. Your genome data is permanently retained and re-analyzed automatically as science advances.
What happens if a clinically significant variant is found?
If a pathogenic or likely-pathogenic variant is identified, it will be clearly flagged in your physician-ready report with clinical context, published evidence, and recommended next steps. We recommend sharing any clinically significant finding with your physician or a genetic counselor, who can guide decisions about surveillance, risk reduction, or cascade testing for family members.
How is this different from a consumer DNA test like 23andMe or AncestryDNA?
Consumer DNA tests use genotyping chips that read less than 0.1% of your genome — a tiny pre-selected set of common variants. They are optimized for ancestry and population-level traits, not clinical genetic findings. The Dante Genome Test sequences 100% of your genome at 30X coverage, the same standard used in clinical diagnostic settings. The two tests are not comparable in scope, methodology, or clinical utility.
How long does it take to get results, and how are they delivered?
Your collection kit ships within 48 hours of ordering. Once your sample arrives at our CLIA-certified laboratory, sequencing and analysis takes 6–8 weeks. Results are delivered securely to your Genome Manager, where you can access your reports, share them with your physician, and receive automatic updates as new findings are validated against your genome.
We work with patient advocacy groups worldwide.
Dante Labs works with patient advocacy groups of any size — for Tamoxifen Response — CYP2D6 and other conditions, rare and common. We support groups in any country, including virtual patient advocacy groups.
We can provide customized reports, group discounts, and packages tailored for your members. Please reach out using the form and we'll be in touch within two business days.
- Custom genomic reports for your members
- Group discounts and tailored packages
- Any country — including virtual groups
- Rare and common conditions covered
Message received.
We'll be in touch within 2 business days. To follow up directly: hello@dantelabs.com
One test.
A lifetime of answers.
One kit, sent to your home. Your entire genome sequenced at the clinical standard used for diagnostic decisions. 200+ physician-ready reports delivered to your Genome Manager in 6–8 weeks — permanent and updated as science advances.
Ships within 48 hours · Results in 6–8 weeks
