April 3, 2026
Biomarkers are measurable features found in cancer cells — including specific proteins, gene mutations, and other molecular changes — that tell doctors important things about a cancer: what type it is, how it is likely to behave, and which treatments are most likely to work. Biomarker and genetic testing have become one of the most important parts of modern cancer care, and the results in your pathology report are guiding your treatment in ways that were not possible even a decade ago.
This section explains the most commonly tested biomarkers and genetic tests across all major cancer types. Each article covers what a specific test looks for, how results are reported, and what they mean for treatment and prognosis.
If you are looking for a plain-language explanation of a specific cancer diagnosis, visit our Diagnosis Guides. If you are looking for the definition of a single medical term, visit our Pathology Dictionary.
New to biomarkers and genetic testing?
An introduction to biomarkers and molecular testing in cancer — what biomarkers are, how they are tested, and how results are used to guide diagnosis, prognosis, and treatment.
Start here if you are new to biomarker testing, or if you want to understand what the molecular testing section of your pathology report actually means.
Understanding genetic testing in cancer
An overview of somatic and germline genetic testing — the difference between testing the tumour’s DNA and testing inherited DNA, what to expect from hereditary genetic testing, and what a result means for you and your family.
Start here if you have been referred for genetic testing, if your oncologist has raised the possibility of a hereditary cancer syndrome, or if you want to understand whether your cancer may have an inherited cause.
Browse by Cancer Type
Breast Cancer
Estrogen receptor (ER) and progesterone receptor (PR) in breast cancer
A guide to hormone receptor testing in breast cancer, including how results are reported and what they mean for hormone-blocking therapy.
You may find this helpful if your breast cancer report includes ER and PR results, an Allred score or H-score, or a recommendation for tamoxifen or an aromatase inhibitor.
A guide to HER2 testing in breast cancer by immunohistochemistry and FISH, including HER2-positive, HER2-low, and HER2-negative results.
You may find this helpful if your breast cancer report includes a HER2 IHC score (0, 1+, 2+, or 3+), a FISH result, or a recommendation for trastuzumab or another HER2-targeted therapy.
A guide to Ki-67 as a proliferation marker in breast cancer, including how the result is expressed as a percentage and what a high or low result means.
You may find this helpful if your breast cancer report includes a Ki-67 index or proliferation rate, or if your doctor used the Ki-67 result to guide decisions about chemotherapy.
BRCA1 and BRCA2 in breast cancer
A guide to BRCA1 and BRCA2 mutations in breast cancer, including the difference between somatic and germline testing, what a positive result means for treatment, and implications for family members.
You may find this helpful if your doctor recommended BRCA testing, your report mentions a BRCA1 or BRCA2 mutation, or you have been referred to a genetic counsellor.
Genomic testing in breast cancer (Oncotype DX, Prosigna, MammaPrint) (coming soon)
A guide to multi-gene expression tests used to estimate recurrence risk and guide chemotherapy decisions in early-stage hormone receptor-positive breast cancer.
You may find this helpful if your doctor ordered an Oncotype DX recurrence score, Prosigna (PAM50) risk score, or MammaPrint result to decide whether chemotherapy is needed.
PD-L1 in triple-negative breast cancer (coming soon)
A guide to PD-L1 testing in triple-negative breast cancer, including the combined positive score (CPS) and its role in selecting patients for immunotherapy.
You may find this helpful if you have triple-negative breast cancer and your report includes a PD-L1 result or a CPS score, or if your doctor discussed pembrolizumab.
Lung Cancer
A guide to EGFR mutation testing in non-small cell lung cancer, including common mutation types (exon 19 deletion, L858R, exon 20 insertion) and EGFR-targeted therapies.
You may find this helpful if your lung cancer report includes EGFR mutation testing results, or if your doctor discussed osimertinib, erlotinib, or another EGFR inhibitor.
ALK rearrangements in lung cancer
A guide to ALK gene rearrangement testing in non-small cell lung cancer, how it is detected by FISH or immunohistochemistry, and ALK-targeted therapies.
You may find this helpful if your lung cancer report includes an ALK FISH result or ALK immunohistochemistry, or if your doctor discussed alectinib, crizotinib, or another ALK inhibitor.
ROS1 rearrangements in lung cancer
A guide to ROS1 gene rearrangement testing in non-small cell lung cancer and the targeted therapies available for ROS1-positive tumours.
You may find this helpful if your lung cancer report includes a ROS1 result, or if your doctor discussed entrectinib or crizotinib for a ROS1-positive cancer.
A guide to KRAS mutation testing in non-small cell lung cancer, with a focus on the KRAS G12C mutation and targeted therapies.
You may find this helpful if your lung cancer report identifies a KRAS mutation, particularly KRAS G12C, or if your doctor discussed sotorasib or adagrasib.
MET alterations in lung cancer
A guide to MET exon 14 skipping mutations and MET amplification in non-small cell lung cancer, and the targeted therapies available for MET-altered tumours.
You may find this helpful if your lung cancer report identifies a MET exon 14 mutation or MET amplification, or if your doctor discussed tepotinib or capmatinib.
A guide to RET gene fusion testing in non-small cell lung cancer and the targeted therapies available for RET-positive tumours.
You may find this helpful if your lung cancer report identifies a RET fusion, or if your doctor discussed selpercatinib or pralsetinib.
A guide to BRAF V600E and other BRAF mutations in non-small cell lung cancer and the targeted therapies available.
You may find this helpful if your lung cancer report identifies a BRAF mutation, or if your doctor discussed dabrafenib and trametinib.
Colorectal Cancer
Mismatch repair (MMR) and microsatellite instability (MSI) in colorectal cancer
A guide to MMR and MSI testing in colorectal cancer, including dMMR versus pMMR, MSI-H versus MSS, Lynch syndrome screening, and the role in immunotherapy selection.
You may find this helpful if your colorectal cancer report includes MMR immunohistochemistry results, MSI testing, MLH1 methylation testing, or a discussion of Lynch syndrome.
KRAS and NRAS mutations in colorectal cancer
A guide to RAS mutation testing in colorectal cancer, why KRAS and NRAS mutations predict resistance to anti-EGFR therapies, and what a wild-type result means.
You may find this helpful if your colorectal cancer report includes KRAS or NRAS mutation results, or if your doctor discussed cetuximab or panitumumab.
BRAF mutations in colorectal cancer
A guide to BRAF V600E mutation testing in colorectal cancer, its role as a prognostic marker and its relevance to Lynch syndrome assessment, and targeted therapy options.
You may find this helpful if your colorectal cancer report identifies a BRAF V600E mutation, or if your doctor discussed encorafenib-based therapy or used BRAF status to interpret your MMR result.
A guide to HER2 amplification and overexpression testing in colorectal cancer and the emerging role of HER2-targeted therapy.
You may find this helpful if your colorectal cancer report includes HER2 testing results, or if your doctor discussed trastuzumab-based therapy for HER2-positive colorectal cancer.
Prostate Cancer
BRCA1 and BRCA2 in prostate cancer (coming soon)
A guide to BRCA1 and BRCA2 mutations in prostate cancer, their role in homologous recombination repair, and the targeted therapies available for BRCA-mutated prostate cancer.
You may find this helpful if your prostate cancer report includes HRR or BRCA mutation testing, or if your doctor discussed olaparib or rucaparib.
Mismatch repair (MMR) and microsatellite instability (MSI) in prostate cancer (coming soon)
A guide to MMR and MSI testing in prostate cancer and the role of dMMR/MSI-H status in selecting patients for immunotherapy.
You may find this helpful if your prostate cancer report includes MMR or MSI testing results, or if your doctor discussed pembrolizumab for MSI-high prostate cancer.
Androgen receptor (AR) and AR splice variants in prostate cancer (coming soon)
A guide to androgen receptor testing and AR-V7 splice variant detection in prostate cancer and what these results mean for hormone therapy decisions.
You may find this helpful if your prostate cancer report mentions androgen receptor expression or AR-V7 testing, or if your doctor is deciding between enzalutamide, abiraterone, and taxane-based chemotherapy.
Melanoma
BRAF mutations in melanoma (coming soon)
A guide to BRAF V600E and V600K mutation testing in melanoma and the targeted therapies available for BRAF-mutated disease.
You may find this helpful if your melanoma report includes BRAF mutation testing, or if your doctor discussed dabrafenib and trametinib or vemurafenib and cobimetinib.
PD-L1 and tumour mutational burden (TMB) in melanoma (coming soon)
A guide to PD-L1 and TMB as predictive biomarkers for immunotherapy response in melanoma.
You may find this helpful if your melanoma report includes PD-L1 testing or tumour mutational burden results, or if your doctor discussed pembrolizumab, nivolumab, or ipilimumab.
KIT mutations in melanoma (coming soon)
A guide to KIT mutations in acral and mucosal melanoma subtypes and the targeted therapies available for KIT-mutated melanoma.
You may find this helpful if your melanoma report identifies a KIT mutation, particularly if you have acral lentiginous or mucosal melanoma.
Thyroid Cancer
BRAF mutations in thyroid cancer
A guide to BRAF V600E mutation testing in papillary thyroid carcinoma, its role as a diagnostic and prognostic marker, and targeted therapy options for advanced disease.
You may find this helpful if your thyroid cancer report includes BRAF mutation testing, or if your doctor discussed BRAF status in relation to risk of recurrence or eligibility for targeted therapy.
RET mutations and fusions in thyroid cancer
A guide to RET mutations in medullary thyroid carcinoma and RET fusions in papillary thyroid carcinoma, and the targeted therapies available for RET-altered thyroid cancers.
You may find this helpful if you have medullary or papillary thyroid carcinoma and your report identifies a RET mutation or fusion, or if your doctor discussed selpercatinib or pralsetinib.
RAS mutations in thyroid cancer
A guide to NRAS, HRAS, and KRAS mutations in thyroid cancer, their role in the diagnosis of indeterminate thyroid nodules, and their prognostic significance.
You may find this helpful if your thyroid fine needle aspiration or surgical report includes RAS mutation testing, or if a RAS mutation was identified on a molecular panel such as ThyroSeq or Afirma.
Gastrointestinal Cancers
HER2 in gastric and gastroesophageal junction cancer (coming soon)
A guide to HER2 testing in gastric and gastroesophageal junction adenocarcinoma, including how HER2 scoring differs from breast cancer and the targeted therapies available.
You may find this helpful if you have stomach or gastroesophageal junction cancer and your report includes HER2 results, or if your doctor discussed trastuzumab or trastuzumab-deruxtecan.
PD-L1 in gastric and gastroesophageal junction cancer (coming soon)
A guide to PD-L1 testing by combined positive score (CPS) in gastric and gastroesophageal junction cancer and its role in immunotherapy selection.
You may find this helpful if your gastric or gastroesophageal junction cancer report includes a PD-L1 CPS result, or if your doctor discussed nivolumab or pembrolizumab.
Mismatch repair (MMR) and microsatellite instability (MSI) in gastrointestinal cancers (coming soon)
A guide to MMR and MSI testing across gastrointestinal cancers including gastric, small bowel, and pancreatic cancers, and the implications for immunotherapy and Lynch syndrome screening.
You may find this helpful if you have a gastrointestinal cancer and your report includes MMR immunohistochemistry or MSI testing results.
KRAS mutations in pancreatic cancer (coming soon)
A guide to KRAS mutation testing in pancreatic ductal adenocarcinoma, why KRAS mutations are nearly universal in this cancer, and emerging KRAS-targeted therapies.
You may find this helpful if you have pancreatic cancer and your report includes molecular testing results, or if your doctor discussed KRAS-targeted treatments or clinical trial eligibility.
BRCA1 and BRCA2 in pancreatic cancer (coming soon)
A guide to BRCA1 and BRCA2 mutations in pancreatic cancer, their role in selecting patients for PARP inhibitor therapy, and implications for family members.
You may find this helpful if you have pancreatic cancer and your report identifies a BRCA1 or BRCA2 mutation, or if your doctor discussed olaparib maintenance therapy.
Gynecologic Cancers
Mismatch repair (MMR) in endometrial cancer
A guide to MMR testing in endometrial carcinoma, its role as a prognostic classifier in the TCGA molecular classification system, Lynch syndrome screening, and immunotherapy selection.
You may find this helpful if you have endometrial cancer and your report includes MMR immunohistochemistry results or mentions dMMR, pMMR, or Lynch syndrome.
POLE mutations in endometrial cancer
A guide to POLE (DNA polymerase epsilon) exonuclease domain mutations in endometrial carcinoma, why POLE-mutated tumours have an excellent prognosis despite high grade, and the role of POLE in the molecular classification of endometrial cancer.
You may find this helpful if your endometrial cancer report identifies a POLE mutation or classifies your tumour as POLE-ultramutated, or if your doctor discussed how the POLE result affects your treatment plan.
A guide to p53 immunohistochemistry and TP53 mutation testing in endometrial carcinoma, what abnormal p53 expression means, and its role in the molecular classification and prognosis of endometrial cancer.
You may find this helpful if your endometrial cancer report includes p53 immunohistochemistry results or TP53 mutation testing, or if your tumour has been classified as p53-abnormal.
BRCA1 and BRCA2 in ovarian cancer
A guide to BRCA1 and BRCA2 mutation testing in high-grade serous ovarian carcinoma, homologous recombination deficiency (HRD) testing, PARP inhibitor therapy, and implications for family members.
You may find this helpful if you have ovarian cancer and your report or genetic testing identifies a BRCA1 or BRCA2 mutation, or if your doctor discussed olaparib, niraparib, or rucaparib.
Homologous recombination deficiency (HRD) in ovarian cancer
A guide to HRD testing in ovarian cancer, what genomic instability scores mean, and how HRD status beyond BRCA mutations affects PARP inhibitor eligibility.
You may find this helpful if your ovarian cancer report includes an HRD score, genomic instability testing, or mentions of BRCA-like status or homologous recombination proficiency.
Blood Cancers
BCR::ABL1 (Philadelphia chromosome) in chronic myeloid leukemia
A guide to the BCR::ABL1 fusion gene and Philadelphia chromosome in CML, how it is detected and monitored, and the tyrosine kinase inhibitors used to treat CML.
You may find this helpful if you have CML and your report includes BCR::ABL1 PCR results, FISH testing, or a karyotype showing the Philadelphia chromosome, or if your doctor discussed imatinib, dasatinib, or nilotinib.
FLT3 mutations in acute myeloid leukemia
A guide to FLT3-ITD and FLT3-TKD mutations in AML, what a high allelic ratio means, and the FLT3-targeted therapies now available.
You may find this helpful if you have AML and your molecular testing identifies a FLT3 mutation, or if your doctor discussed midostaurin, quizartinib, or gilteritinib.
NPM1 mutations in acute myeloid leukemia (coming soon)
A guide to NPM1 mutations in AML, why NPM1-mutated AML has a relatively favorable prognosis, and how NPM1 is used to monitor for residual disease after treatment.
You may find this helpful if you have AML and your molecular testing identifies an NPM1 mutation, or if your doctor is using NPM1 PCR to monitor your response to treatment.
IDH1 and IDH2 mutations in acute myeloid leukemia (coming soon)
A guide to IDH1 and IDH2 mutations in AML, their prognostic significance, and the targeted IDH inhibitors now available.
You may find this helpful if you have AML and your molecular testing identifies an IDH1 or IDH2 mutation, or if your doctor discussed ivosidenib or enasidenib.
JAK2 mutations in myeloproliferative neoplasms
A guide to the JAK2 V617F mutation and JAK2 exon 12 mutations in polycythemia vera, essential thrombocythemia, and primary myelofibrosis, and the role of JAK inhibitors in treatment.
You may find this helpful if you have polycythemia vera, essential thrombocythemia, or myelofibrosis and your report includes JAK2 mutation testing results, or if your doctor discussed ruxolitinib or fedratinib.
Calreticulin (CALR) mutations in myeloproliferative neoplasms (coming soon)
A guide to CALR mutations in essential thrombocythemia and primary myelofibrosis, what type 1 and type 2 CALR mutations mean, and their prognostic significance.
You may find this helpful if you have essential thrombocythemia or myelofibrosis and your report identifies a CALR mutation, or if JAK2 testing was negative and CALR was tested as an alternative.
TP53 mutations in blood cancers
A guide to TP53 mutations in leukemia, lymphoma, and myeloma, why TP53 mutations are associated with more aggressive disease and treatment resistance, and emerging therapies targeting p53-deficient cancers.
You may find this helpful if you have a blood cancer and your molecular testing identifies a TP53 mutation, or if your doctor discussed how TP53 status affects your prognosis or treatment options.
Chromosome 17p deletion and TP53 mutation in chronic lymphocytic leukemia (coming soon)
A guide to 17p deletion and TP53 mutation testing in CLL, why these findings predict resistance to standard chemotherapy, and the targeted therapies recommended for high-risk CLL.
You may find this helpful if you have CLL and your FISH panel or molecular testing identifies a 17p deletion or TP53 mutation, or if your doctor discussed ibrutinib, venetoclax, or acalabrutinib.
IGHV mutation status in chronic lymphocytic leukemia (coming soon)
A guide to IGHV mutation status in CLL, the difference between mutated and unmutated IGHV, and why this test is one of the strongest prognostic markers in CLL.
You may find this helpful if you have CLL and your report includes IGHV mutation status, or if your doctor discussed how IGHV status affects your long-term prognosis and treatment approach.
Pan-Cancer Biomarkers
Tumour mutational burden (TMB)
A guide to tumour mutational burden as a pan-cancer biomarker for immunotherapy response, how it is measured, and what a high or low TMB result means.
You may find this helpful if your cancer report includes a TMB result expressed as mutations per megabase, or if your doctor discussed TMB as part of immunotherapy eligibility assessment.
Mismatch repair (MMR) and microsatellite instability (MSI) across cancer types
A guide to dMMR and MSI-H as tumour-agnostic biomarkers for immunotherapy, covering how MSI-H leads to pembrolizumab eligibility regardless of cancer type and what this means for Lynch syndrome screening.
You may find this helpful if your cancer report identifies dMMR or MSI-H and your doctor discussed immunotherapy eligibility based on these results regardless of the primary cancer site.
An overview of PD-L1 as a biomarker for immune checkpoint inhibitor therapy across cancer types, including how it is tested, how different scoring systems work, and the limitations of PD-L1 as a predictive marker.
You may find this helpful if your cancer report includes a PD-L1 result and you want to understand how PD-L1 testing works across different cancer types and treatment contexts.
A guide to NTRK1, NTRK2, and NTRK3 fusions as tumour-agnostic biomarkers for TRK inhibitor therapy, found across many cancer types, including lung, thyroid, colorectal, salivary gland, and others.
You may find this helpful if your cancer report identifies an NTRK fusion, or if your doctor discussed larotrectinib or entrectinib as tumour-agnostic targeted therapy options.
Understanding next-generation sequencing (NGS) in cancer (coming soon)
An overview of next-generation sequencing panels used in cancer diagnosis and treatment planning, including what NGS tests look for, how results are reported, and what variants of uncertain significance mean.
You may find this helpful if your cancer report includes an NGS or comprehensive genomic profiling result, or if you received a report from a test such as FoundationOne, MSK-IMPACT, or a hospital-based molecular tumour board panel.
Liquid biopsy and circulating tumour DNA (ctDNA) (coming soon)
A guide to liquid biopsy and ctDNA testing, how tumour DNA shed into the bloodstream is detected and analysed, and the uses of ctDNA in cancer diagnosis, monitoring, and detecting residual disease.
You may find this helpful if your doctor ordered a blood-based cancer test such as a Guardant360, FoundationOne Liquid, or similar ctDNA panel, or discussed liquid biopsy as an alternative or complement to tissue testing.
Don’t see your biomarker listed?
New biomarker guides are added regularly as testing evolves. In the meantime, our Pathology Dictionary defines many biomarker-related terms, and Osler, our pathology chatbot, can help explain specific biomarker results from your report in plain language.
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