The short answer
Tumor testing can find a change a specific drug is built to block. Here is what that means, what it does not mean, and what to ask next.
A mutation is targetable only when an approved drug is designed to block it; most changes on a report are simply present and change nothing.
Tumor testing can hint at an inherited variant, but confirming it needs a separate germline test and genetic counseling, and the answer affects blood relatives.
Some approvals are tumor-agnostic, based on what the tumor has rather than where it started, such as larotrectinib and entrectinib for NTRK fusions and pembrolizumab for MSI-high or TMB-high tumors.
Full results commonly take about two to three weeks, and waiting is often worth it when it is safe, because the wrong first treatment can close off later options.
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The full explanation.
What "targetable" actually means
A tumor report can list many genetic changes. Only a few of them change what happens next. A change is called targetable, or actionable, under two conditions. A specific drug has been built to block what that change does. And there is evidence the drug helps people whose tumors carry it.
Everything else on the report is simply present. Some changes are harmless. Some are "variants of uncertain significance," meaning no one yet knows what they do. A long report is not the same as a report full of options. It is fair to ask your team to point at the line that changes the plan.
Somatic or inherited: two different questions
Most changes found in a tumor are somatic. They arose inside the cancer cells during your life, and you cannot pass them to your children. But tumor testing sometimes turns up a change that looks inherited. That means it has been in every cell of your body since birth.
The National Cancer Institute notes that tumor testing can sometimes detect inherited changes as well. It does not replace testing for inherited cancer risk. Confirming it takes a separate germline test, usually on blood or saliva, with genetic counseling alongside.
This is the point where the result stops being only about you. Genetic findings also say something about blood relatives. They may want their own testing and earlier screening.
Targets that already have drugs
Some of the best-known matches include EGFR changes in lung cancer, treated with drugs such as osimertinib. Others are ALK and ROS1 fusions, and BRAF V600E. KRAS G12C is another, where sotorasib was approved in 2021. HER2 is on the list too.
A few approvals are tumor-agnostic. That means they go by what the tumor has, rather than where it started. Larotrectinib and entrectinib are approved for solid tumors with an NTRK gene fusion. Pembrolizumab is approved for MSI-high or mismatch-repair-deficient tumors. In 2020 it was also approved for tumors with a high mutational burden, defined as ten or more mutations per megabase, after earlier treatment.
Why the wait is usually worth it
Full results commonly take about two to three weeks. The wait is hard. It is often the better choice, though, because starting the wrong treatment first can matter. Some sequences close off later options, or add side effects for no benefit.
If your disease is stable, waiting is usually safe. If you are very unwell, or symptoms are moving fast, your team may start something now and adjust when results land. Ask plainly whether it is safe for you to wait.
Tissue, blood, or both
Testing can use tumor tissue from a biopsy or surgery. It can also use a blood sample, called a liquid biopsy, which picks up tumor DNA circulating in the blood. Liquid biopsy is quicker. It is useful when a tumor is risky or hard to reach with a needle.
One limitation matters a great deal. A negative liquid biopsy does not rule a mutation out. Some tumors shed little DNA into the blood. The FDA's own approval notice for sotorasib puts it directly: if no mutation is detected in a plasma specimen, the tumor tissue should be tested.
A match, but no approval for your cancer
Sometimes a target is found, and the matching drug is approved only for a different cancer type. Several options open up then. Off-label use is one; insurers often deny it at first, and it can sometimes be won on appeal with a letter from your oncologist. Basket trials are another; they enrol people by biomarker rather than by organ. Expanded access is a third, where your doctor asks the FDA and the drug company for a drug still in testing.
Resistance, and testing again later
Targeted drugs usually stop working eventually. The NCI explains why. Sometimes the target itself changes, so the drug can no longer grip it. Sometimes cancer cells find new ways to grow that do not depend on the target.
That is not the end of the road. A repeat biopsy at the time of progression can show the new mechanism. Sometimes that mechanism has a drug of its own.
Keeping it in proportion
A targetable mutation is genuinely good news. It is not a cure. Responses can be strong and long-lasting, but the cancer is being controlled rather than removed. Tumors are also mixed, so a drug may kill some cells and leave others.
Most people's tumors do not have a targetable change. That is disappointing. It does not mean treatment has run out.
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Words to know
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Common questions
My report lists several mutations. Does that mean several treatment options?
Usually not. Most listed changes are harmless, or are variants of uncertain significance where no one yet knows what they do. Only a change with a matching drug and evidence behind it alters the plan. Ask your team which single line on the report, if any, changes what they would recommend.
Does a mutation found in my tumor mean my children are at risk?
Usually not. Most tumor changes are somatic, meaning they arose in the cancer cells during your life and are not passed on. Occasionally tumor testing flags something that looks inherited. That needs a separate germline test, usually on blood or saliva, with genetic counseling, before anything is said about family risk.
My blood test came back negative. Is that settled?
Not necessarily. Some tumors shed very little DNA into the blood, so a liquid biopsy can miss a change that is really there. The FDA's approval notice for sotorasib states that if no mutation is detected in a plasma specimen, the tumor tissue should be tested. Ask whether tissue testing is a reasonable next step.
The drug matching my mutation is approved for a different cancer. What now?
There are several routes. Off-label use is sometimes possible, though insurers often deny it at first and an appeal with a letter from your oncologist can help. Basket trials enrol people by biomarker rather than by organ. Expanded access lets your doctor request a drug still in testing from the FDA and the manufacturer.
The drug worked, then stopped. Is that the end?
Targeted drugs usually stop working eventually, either because the target itself changes or because the cancer finds a route to grow that does not use the target. A repeat biopsy at the point of progression can show which of those happened, and sometimes the new mechanism has a drug of its own.
Questions to ask your doctor
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Written by: Cancer ExplainedSources last checked: 2026-07-30 what this meansLast updated: 2026-08-10Next planned review: 2027-01-30
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How this page was created
Cancer Explained does not originate medical claims. Every page restates guidance already published by the National Cancer Institute, the CDC, the USPSTF and the FDA, in plain language, with the source cited so you can check the original yourself. AI does the translating and organizing; automated checks test claims, citations, clarity and safety before anything publishes. We do not employ clinicians and do not intend to — our work is translation and navigation, not clinical judgment. Nothing here is personal medical advice, and no page can account for your particular situation.
Editorial status: Source checked — This page was written with AI assistance and checked line by line against the sources listed on it. That confirms the sources support what the page says. It is not a medical review, and it does not confirm the page is complete or right for your situation.
Human medical review: not completed. Pages here are not signed off by a clinician before they publish. That is not an oversight we are quietly working around: we restate published guidance and cite it, so the authority belongs to the source rather than to us, and every page names where its claims come from — you can verify us instead of trusting us. Where a volunteer clinician has reviewed a page, their name and credentials appear on it; where no name appears, no clinician has checked it. We are glad to have reviewers and are recruiting them, and we do not hold pages back waiting for one. Use this site to understand your situation and to ask better questions of the people treating you.
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