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.
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:
Different biomarkers serve different purposes in cancer care. Many serve more than one purpose at the same time.
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.
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.
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.
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.
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:
Results from these tests are reported in the molecular testing, biomarker testing, or ancillary studies section of your pathology report.
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.
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:
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.