What is a Biomarker?

by Jason Wasserman MD PhD FRCPC
March 27, 2026

If you have been diagnosed with cancer, you have probably come across the word “biomarker” — in your pathology report, in conversations with your oncologist, or in articles about your cancer type. It is one of the most important words in modern cancer care, and understanding what it means will help you make sense of much of what happens during diagnosis and treatment. A biomarker is simply a measurable feature of a cancer — something that can be detected and measured in a laboratory — that tells doctors something useful about the cancer: what type it is, how it is likely to behave, and which treatments are most likely to work. Biomarker testing has fundamentally changed the way cancer is treated, shifting the approach from treating all cancers of the same organ the same way to tailoring treatment to the individual characteristics of each person’s tumour.

What kinds of biomarkers are there?

Biomarkers in cancer come in several forms. Some are proteins found on the surface of or inside cancer cells. Some are specific changes — called mutations — in the DNA of cancer cells. Some are the number of copies of a gene. Some reflect how actively a gene is being used. Despite this variety, all biomarkers share one characteristic: they can be detected with laboratory tests, and their results carry clinical meaning.

Here are the main categories you are likely to encounter:

Protein biomarkers. Some cancers produce specific proteins in abnormal amount, or display abnormal proteins on their surface. Testing for these proteins — usually using a laboratory method called immunohistochemistry (IHC) — helps pathologists determine the type of cancer and whether certain treatments are likely to work. The estrogen receptor in breast cancer and HER2 in breast, gastric, and colorectal cancer are well-known examples.
DNA mutation biomarkers. Cancers accumulate changes in their DNA called mutations. Some mutations are the direct drivers of cancer growth — they switch on pathways that tell cells to keep dividing. Others predict whether targeted drugs will work or fail. KRAS mutations in colorectal and lung cancer, EGFR mutations in lung cancer, and BRAF mutations in melanoma and colorectal cancer are common examples. These are detected using DNA testing methods such as PCR or next-generation sequencing.
Gene rearrangement biomarkers. Sometimes two separate genes become abnormally joined, creating a fusion gene. The resulting fusion protein is often permanently switched on and drives cancer growth. ALK fusions and ROS1 fusions in lung cancer are examples. These are detected by FISH testing or RNA-based sequencing.
Gene copy number biomarkers. Normally, every cell carries two copies of each gene. In some cancers, a gene is amplified — many extra copies are produced — leading to an overabundance of the protein it encodes. HER2 amplification is the best-known example.
DNA repair biomarkers. Some cancers develop because a cell’s system for repairing DNA errors has broken down. MMR deficiency (dMMR) and microsatellite instability (MSI-H) reflect this kind of repair failure. Tumours with these features tend to respond well to immunotherapy. POLE mutations cause a different kind of repair failure, leading to a very large number of mutations but, paradoxically, excellent outcomes in endometrial cancer.
Broad genomic measures. Tumour mutational burden (TMB) measures the overall number of mutations across the tumour’s genome. A high TMB is associated with better response to immunotherapy across many cancer types.

What do biomarkers actually tell doctors?

Different biomarkers serve different purposes in cancer care. Many serve more than one purpose at the same time.

Confirming the diagnosis

Some biomarkers help confirm which type of cancer a patient has, especially when different cancers can look similar under the microscope. Protein markers detected by immunohistochemistry are particularly useful here — they can identify where a cancer originated, or distinguish between two cancers that look alike but require very different treatments.

Predicting how the cancer will behave

Some biomarkers predict the likely course of a cancer — whether it is likely to grow quickly, spread, or come back after treatment. A biomarker that carries this kind of information is called a prognostic biomarker. Ki-67 in breast cancer (a measure of how quickly cancer cells are dividing) and POLE mutations in endometrial cancer (which identify a group with an excellent prognosis despite high-grade appearance) are examples.

Predicting whether a specific treatment will work

This is perhaps the most important role biomarkers play today. A biomarker that predicts whether a specific treatment is likely to be effective — or ineffective — is called a predictive biomarker. KRAS mutations in colorectal cancer predict that anti-EGFR drugs will not work. HER2 overexpression predicts that HER2-targeted drugs will work. PD-L1 expression and MMR deficiency predict better responses to immunotherapy. These findings directly determine which drugs are offered and which are withheld.

Guiding targeted therapy

A special category of predictive biomarkers is that which identifies the specific molecular target that a cancer depends on for its growth. These “driver mutations” are the reason the cancer exists and persists — and they are also the cancer’s vulnerability. EGFR mutations in lung cancer, BRAF V600E mutations in melanoma, and BCR::ABL1 fusions in chronic myeloid leukaemia are all examples of driver mutations that have been matched to drugs specifically designed to block them. Testing for these mutations is now essential before starting treatment, because these drugs only work if the target is present.

How is biomarker testing done?

Most biomarker testing in cancer is done on tumour tissue — the sample collected during a biopsy or during surgery to remove a tumour. In most cases, the same tissue sample used to make the diagnosis is also sufficient for biomarker testing. No additional procedure is usually needed.

Several types of laboratory tests are used:

Immunohistochemistry (IHC) uses specially designed proteins called antibodies to stain tissue sections for specific proteins. A pathologist examines the stained tissue under a microscope and assesses the result. IHC is fast, widely available, and is the method used for many of the most common biomarker tests.
Fluorescence in situ hybridization (FISH) directly examines the genes within tumour cells to detect amplifications, deletions, or rearrangements. It is often used to confirm borderline IHC results or to detect gene fusions.
PCR (polymerase chain reaction) amplifies specific regions of the tumour’s DNA to detect mutations at known locations. It is fast and sensitive, and is commonly used for well-characterised mutations such as EGFR or KRAS.
Next-generation sequencing (NGS) reads large sections of the tumour’s DNA — or sometimes its RNA — simultaneously, allowing many genes and many types of alterations to be assessed in a single test. Comprehensive NGS panels have become the standard approach in many cancer types, providing a detailed molecular picture of the tumour in one test run.

Results from these tests are reported in the molecular testing, biomarker testing, or ancillary studies section of your pathology report.

Tumour biomarkers versus inherited genetic tests

A very common source of confusion is the distinction between biomarker testing performed on the tumour itself and genetic testing performed on a blood or saliva sample to detect inherited mutations. These are fundamentally different tests, performed for different reasons, and the results have very different implications for the patient and their family.

When a biomarker result is found in tumour tissue, it reflects a change that occurred within the cancer cells — not necessarily in the rest of the body, and not necessarily something that was inherited. Most tumour biomarkers are what scientists call somatic changes: they arose during a person’s lifetime, only within the cancer cells. They cannot be passed on to children.

Inherited genetic testing, on the other hand, looks for mutations that are present in every cell of the body — including cells that could be passed to future generations. These hereditary mutations can raise the lifetime risk of certain cancers and have implications for biological relatives.

The Understanding Genetic Testing in Cancer article in this section explains the difference between somatic and germline testing in more detail and walks through what to expect from hereditary genetic testing.

What happens when multiple biomarker results come back at once?

With the widespread use of comprehensive NGS panels, patients increasingly receive reports listing many biomarker results at once — some relevant to immediate treatment decisions, some informative for future planning, and some of uncertain significance. This can feel overwhelming.

A few principles help make sense of multiple results:

Not every result changes your treatment right now. Some results are informative for monitoring or for future decisions if the cancer progresses. Ask your oncologist which results are relevant to your current situation.
Results interact with each other. In some cancers, a combination of biomarker results — for example, RAS status plus tumour sidedness in colorectal cancer, or MMR status plus POLE status in endometrial cancer — determines the overall treatment approach. No single result should be read in isolation.
Variants of uncertain significance are common. A variant of uncertain significance (often abbreviated VUS) is a DNA change whose clinical meaning is not yet known. It is not the same as a confirmed harmful mutation, and it should not be treated as one. Your oncologist or genetic counsellor will explain whether any VUS results in your report are relevant to your care.

Why biomarker testing matters for you

Twenty years ago, cancer treatment was largely determined by where in the body a cancer started — the organ of origin — and how far it had spread. Biomarker testing has shifted this fundamentally. Two people with cancers that look identical under the microscope and are at the same stage may now receive completely different treatments based on their tumour’s molecular profile, and have dramatically different outcomes as a result.

For patients, understanding that your cancer has specific molecular characteristics — and that those characteristics are guiding your treatment — can make a meaningful difference in how you participate in your own care. Knowing what your biomarker results mean allows you to ask better questions, understand why specific drugs are being recommended or avoided, and make more informed decisions about your treatment options, including clinical trials that may be relevant to your molecular profile.

The articles in this section cover the most commonly tested biomarkers across all major cancer types. Use the navigation page to find the article most relevant to your cancer type and the specific result in your report.

Questions to ask your doctor

Which biomarker tests were performed on my tumour, and are all the results back?
Which results are directly relevant to my treatment decisions right now?
Are there any biomarker results that suggest I should see a genetic counsellor?
Are there any results labelled as variants of uncertain significance — and if so, what does that mean for me?
Are there clinical trials I might be eligible for based on my biomarker results?
Should my biomarker testing be repeated if my cancer comes back or progresses?