Biomarker Explainers

A gene name alone in a report rarely clarifies much. What determines a biomarker's therapeutic meaning is the exact variant, the tumor type, and the stage of treatment you are at.

In your NGS report, what does an EGFR mutation mean?

Seeing “EGFR mutation” on a molecular (NGS) report means that the tumor carries an alteration in the EGFR gene, and that this alteration may be one of the factors driving the cancer's growth. In many cancers — most often non-small cell lung cancer (NSCLC) — this finding can open the door to targeted therapies. But the gene name alone does not clarify much: whether your specific EGFR result is truly actionable depends on the exact variant, the type of cancer, and the stage of treatment you are at. Clarifying exactly that is the purpose of a molecular second opinion.

1Which EGFR mutations are actionable?

Not every EGFR mutation behaves the same way, and that is the crux of the matter. The classic “activating” mutations — exon 19 deletions and the L858R point mutation — are the ones that predict response to EGFR tyrosine kinase inhibitors (TKIs) in NSCLC and are supported by guidelines. Exon 20 insertions are a different situation: they generally do not respond to these classic TKIs and may instead require specific agents. Other EGFR alterations can fall into a gray zone — variants of uncertain significance (VUS) whose meaning is not yet clear. A second opinion clarifies which group your result belongs to, because what matters is the exact variant, not the word “EGFR” alone.

2Does an EGFR result mean the same thing in every cancer?

No. How actionable it is depends on the tumor type. In NSCLC, an activating EGFR mutation sits on a well-mapped targeted-therapy pathway, whereas the same gene alteration may carry a different meaning — or no established meaning at all — in a different cancer. The result should not be read directly from the gene name; it should be interpreted in the context of your specific cancer and treatment line.

3Why should you get a second opinion on an EGFR result?

In practice, a molecular second opinion does four things with an EGFR finding. It identifies the exact variant (among an exon 19 deletion, L858R, an exon 20 insertion, a rare alteration, or a VUS). It re-grades how actionable that variant is for your tumor type and treatment line using the AMP/ASCO/CAP framework. It maps the resistance you may encounter down the line (for example, T790M, C797S, or MET-driven escape) and what should be tested for at progression. It also checks your eligibility for molecular and clinical trial matching. All of this is brought together in a single written, physician-signed report for your treating oncologist.

In your NGS report, what does a KRAS mutation mean?

A KRAS mutation on your NGS (next-generation sequencing) report means the tumor carries an alteration in the KRAS gene — one of the most common drivers in cancer, particularly frequent in non-small cell lung cancer (NSCLC) and colorectal cancer. The single most important detail is which exact variant you have. A KRAS G12C mutation may make you eligible for a guideline-recognized class of targeted inhibitors, while many other KRAS variants, such as G12D or G12V, do not yet have an approved targeted drug and are typically managed through standard therapy or a clinical trial. Whether any KRAS finding is actionable depends on your tumor type and treatment line, and the decision always belongs to your treating oncologist. Think of this page not as a diagnosis, but as decision support to help you ask sharper questions.

1Is a KRAS mutation good or bad, and is it treatable?

A KRAS mutation is not automatically good or bad news; what it means depends almost entirely on which variant you have and which cancer you have. The most therapeutically important variant is KRAS G12C. In advanced NSCLC, a guideline-recognized class of oral KRAS G12C inhibitors is available, typically used after first-line therapy, and this is reflected in the NCCN and ESMO guidelines. In colorectal cancer, the same G12C variant is much rarer, and the guideline-supported approach there combines a KRAS G12C inhibitor with an anti-EGFR antibody, because the inhibitor alone appears less effective in colorectal tumors. For many other variants — such as G12D, G12V, G12A, or G13D — there is currently no approved targeted therapy in most tumor types, so treatment typically relies on standard chemotherapy, immunotherapy, or a clinical trial. Here, a second opinion is most useful for confirming the exact variant, assessing the tumor-type context, and checking whether a suitable trial exists.

2I have KRAS G12C. Does that mean I'm eligible for targeted therapy?

Possibly, but that depends on your tumor type and treatment line, which is why confirmation matters. In advanced NSCLC, a KRAS G12C mutation typically opens the door to a guideline-recognized class of KRAS G12C inhibitors, usually considered after a first-line therapy such as immunotherapy or chemotherapy, rather than at the very start. In colorectal cancer, a KRAS G12C inhibitor is used not alone but combined with an anti-EGFR antibody, again typically after prior treatment lines. Eligibility also depends on the result being confirmed by a validated test, your overall health status, your prior treatments, and other findings in your report. Because timing and the right drug pairing are critical, a common reason patients seek a second opinion is to make sure the correct treatment line and combination are being considered. This is general guideline-level information, not a prescription; your treating oncologist decides whether a targeted option is right for you and when it should be given.

3My report shows TP53, STK11, or KEAP1 alongside KRAS. Why does that matter?

When KRAS appears together with TP53, STK11, or KEAP1, these co-occurring mutations can change how physicians view immunotherapy — which is exactly why they are worth raising in a second opinion. In NSCLC, tumors carrying KRAS together with STK11 or KEAP1 are often described as immunologically “cold” and, in published series, tend to derive less benefit from immune checkpoint inhibitors and carry a less favorable prognosis. A TP53 co-mutation has its own, context-dependent effects. Importantly, these patterns are signals, not on/off switches: on their own they do not turn a treatment on or off, and guidelines still individualize the decision. This is exactly the kind of nuance a second opinion is built to handle: reading the whole co-mutation picture together rather than KRAS in isolation, and checking whether immunotherapy, chemotherapy, a combination, or a trial best fits your tumor. Your treating oncologist remains the decision-maker.

What does an ALK fusion in your NGS report mean, and when does a second opinion help?

An ALK rearrangement on your NGS (next-generation sequencing) report — also called an ALK fusion — means that your tumor, most often non-small cell lung cancer (NSCLC), carries an ALK gene fusion that can drive its growth. This is generally one of the most actionable findings in lung cancer: in advanced NSCLC it is matched, at guideline category 1 level, with a drug class called ALK tyrosine kinase inhibitors (ALK TKIs). An ALK fusion is not the same thing as an ALK point mutation, and how it was detected genuinely matters, because some DNA-only panels can miss fusions that RNA testing would catch. Actionability again depends on your tumor type and treatment line, and the decision always belongs to your treating oncologist.

1What does an ALK rearrangement or ALK fusion actually mean?

It generally means your tumor carries a structural rearrangement in which part of the ALK gene has fused to another gene, producing an abnormal protein that can drive cancer growth. This is most common in NSCLC, where ALK fusions occur in roughly 3 to 7 percent of cases, and the most frequent partner gene is EML4 (you may see a label such as EML4-ALK on your report). The key distinction is this: an ALK fusion or rearrangement is not the same thing as an ALK point mutation. It is the fusion that is matched with ALK-targeted therapy; ALK point mutations, by contrast, more often emerge as a resistance alteration during treatment. In advanced NSCLC, an ALK fusion is generally treated as a strong, guideline-level (category 1) actionable driver and matched to the ALK tyrosine kinase inhibitor class. ALK fusions can also be seen in a few other cancers (for example, certain lymphomas and rarer tumors), but the evidence and degree of actionability vary by tumor type, so the same finding does not carry the same weight everywhere.

2My ALK results don't agree (FISH, IHC, and NGS) — or could the panel have missed it?

Discordant ALK results are a recognized issue, and yes, a DNA-only NGS panel can miss some true ALK fusions — which is why methodology matters. ALK status is commonly tested three ways: FISH (looks for the rearrangement in DNA), IHC (looks for the ALK protein), and NGS (sequencing). These do not always agree, and published case series describe tumors that read positive by one method and negative by another. An important practical point: in real-world datasets, RNA-based NGS detects meaningfully more ALK fusions than DNA-only NGS alone — roughly 15 to 18 percent more — because some fusion breakpoints and partners are difficult to capture from DNA. So if your clinical picture strongly suggests an ALK-driven tumor (for example, a never- or light-smoker with lung adenocarcinoma) but a DNA panel comes back negative, it is reasonable to ask your oncologist whether an orthogonal confirmation — RNA fusion testing, IHC, or FISH — is warranted before concluding ALK is absent. The reverse also matters: a positive result from a single method, or an unusual fusion partner, may deserve closer scrutiny before being treated as fully actionable. This is exactly the kind of test-methodology question a second opinion can help resolve.

3If I'm ALK-positive, what are my options, and what happens when the drug stops working?

If you have an ALK fusion in advanced NSCLC, the guideline-preferred first-line approach is generally not chemotherapy or immunotherapy alone, but a drug from the ALK tyrosine kinase inhibitor (ALK TKI) class. Several generations of ALK TKIs are available, and current guidelines favor newer-generation ALK inhibitors as initial therapy for many patients because they tend to work better and control brain metastases more effectively; the specific agent and sequencing are decisions your oncologist makes for your situation. Over time, cancer can develop resistance, and this is where NGS becomes useful a second time. At progression, retesting the tumor tissue or a blood-based (liquid biopsy) NGS test can sometimes reveal the mechanism — for example, an acquired ALK point mutation such as G1202R, one of the resistance alterations seen more often after second-generation ALK inhibitors. Identifying this mechanism can help your oncologist choose the next ALK inhibitor in sequence, because different resistance mutations respond differently to different drugs. The honest bottom line: not every ALK report is equally actionable, sequencing of treatments is individualized, and none of this replaces the judgment of your treating oncologist.

In your NGS report, what does a BRAF V600E mutation mean?

A BRAF V600E mutation means your tumor carries a specific alteration at codon 600 of the BRAF gene that keeps an important growth signaling pathway (the MAPK pathway) switched on. It is the most common and best-understood BRAF variant, and in many cancers it is a recognized, guideline-level target for BRAF-plus-MEK inhibitor therapy (a drug class). Two things determine whether it is actionable for you: your exact tumor type and your exact BRAF variant. V600E (called Class I) behaves very differently from non-V600 (Class II/III) BRAF mutations, where classic BRAF inhibitors can be ineffective — or can even switch the pathway on rather than blocking it. In the large majority of cases, BRAF V600E is a somatic (tumor-specific) alteration rather than an inherited one. This page explains what the result means and where a second reading of your report may change the plan. This content is decision support, not a diagnosis, and the final decision belongs to your treating oncologist.

1Is BRAF V600E targetable, and does that depend on my cancer type?

Yes, BRAF V600E is one of the most actionable findings on an NGS report, but how it is treated depends heavily on your tumor type and treatment line. In melanoma, BRAF V600E-positive disease is a classic indication for combined BRAF-plus-MEK inhibition (a drug class), and the same combination is recognized at guideline level in BRAF V600E non-small cell lung cancer and BRAF V600E anaplastic thyroid cancer. There is also a tissue-agnostic pathway: in 2022, the US FDA granted accelerated approval, based on basket trials, to a BRAF-plus-MEK inhibitor combination for a range of BRAF V600E solid tumors that had progressed on prior therapy and had no satisfactory alternative. Colorectal cancer is the key exception. Here, a BRAF inhibitor alone works poorly because the tumor reactivates the pathway through EGFR; this is why the established, guideline-level first-line standard for BRAF V600E metastatic colorectal cancer combines a BRAF inhibitor with an anti-EGFR antibody and fluorouracil-based chemotherapy. In other words, the same mutation can mean a single targeted combination in one cancer and a different, multi-drug strategy in another. Verify the tumor-type-specific recommendation against current NCCN and ESMO guidelines and the OncoKB BRAF page, and keep in mind that actionability also depends on whether you are in the first line or a later line of treatment.

2My report says BRAF but doesn't say V600E (a non-V600 / Class II/III mutation). Is that the same thing?

No, and this distinction is one of the strongest reasons to have a BRAF result read a second time. Not all BRAF mutations behave like V600E. Scientists divide them into three classes. Class I is V600E and its close relatives (such as V600K); these switch the pathway on by themselves, independent of upstream RAS signaling. Class II comprises non-V600 mutations that signal as active BRAF dimers. Class III has little or no kinase activity of its own and depends on upstream RAS signaling. Treatment implications differ markedly. Classic BRAF inhibitors designed for V600E can be ineffective against many non-V600 variants, and in some situations BRAF inhibitors can paradoxically switch the MAPK pathway on rather than blocking it. For Class II/III BRAF mutations, there is currently no broadly approved targeted standard the way there is for V600E, and management is far less settled. If your report simply says “BRAF mutation,” or names a non-V600 alteration such as a fusion, an in-frame insertion or deletion, or a codon other than 600, it is worth confirming the exact variant and class before assuming a V600E-style treatment applies.

3Does BRAF V600E mean my cancer is hereditary or more aggressive, and what is the prognosis?

In the large majority of cases, BRAF V600E is a somatic mutation — meaning it arose in the tumor itself, is not inherited, generally is not passed from parent to child, and a tumor-only NGS result does not mean your relatives carry it. Whether it points to more aggressive disease depends on the cancer type and is not a verdict on your particular result. In some settings, such as papillary thyroid cancer and BRAF V600E metastatic colorectal cancer, the literature associates this mutation with, on average, more aggressive biology, but this is a statistical pattern across groups, not a prediction for any one person, and the data carry some debate. Equally important, the presence of BRAF V600E also opens targeted options that did not exist a decade ago. Prognosis is shaped by stage, tumor type, overall health, and treatment — not by a single word, “BRAF,” on a report. If hereditary risk concerns your family, that is a separate question and is best addressed through dedicated germline testing and genetic counseling, not by inference from a tumor NGS panel.

In your NGS report, what does HER2 (ERBB2) mean?

On an NGS report, HER2 (also written ERBB2) can refer to one of three different things, and they are not interchangeable: gene amplification (extra copies of the gene), an activating mutation (a change in the gene sequence, often an exon 20 insertion in lung cancer), or protein overexpression measured by a separate stain. Which one your report shows matters a great deal, because the established, guideline-level threshold for most anti-HER2 therapies is generally a protein-level result confirmed by the correct test (IHC and/or FISH) or a specific DNA-level finding — not RNA overexpression alone. Whether any HER2 finding is actionable depends heavily on your tumor type and treatment line. Some HER2 findings open a clear, established treatment pathway; others are not actionable today. The decision belongs to your treating oncologist.

1What is the difference between HER2 amplification, an ERBB2 mutation, and HER2 overexpression?

These are three distinct biological findings measured in different ways, and your NGS report may show one without the others. HER2 amplification means the tumor has extra copies of the ERBB2 gene (a copy-number gain), and an NGS panel derives this from DNA. An ERBB2 (HER2) mutation is a change in the gene's sequence; the most common in lung cancer is an exon 20 in-frame insertion that can activate the receptor independent of copy number. HER2 protein overexpression is the amount of HER2 protein on the cell surface, and DNA sequencing does not measure it at all — it is read by immunohistochemistry (IHC) and scored as 0, 1+, 2+, or 3+. Gene amplification is the main driver of protein overexpression, and the two usually track together, though not always, which is why a confirmatory protein or FISH test is often required. An important caveat: if your report flags HER2 based on RNA expression alone, this is generally not, by itself, the approved actionability threshold and should be confirmed by IHC and/or FISH. Guideline frameworks such as NCCN and ESMO define HER2 positivity by tumor type using these protein and in-situ hybridization criteria.

2Is the HER2 (ERBB2) finding on my report actionable, and which cancers does anti-HER2 therapy apply to?

It may be, but actionability depends on your tumor type, your treatment line, and which HER2 finding you have — a HER2 result is never automatically a treatment plan. As a class, anti-HER2 therapy — including antibody-drug conjugates that deliver chemotherapy directly to HER2-positive cells — is most firmly established in HER2-positive breast and gastric/gastroesophageal cancer and has long been part of guideline-based care. Its role is expanding into other tumor types: activating ERBB2 mutations in NSCLC are a recognized target, and HER2 amplification or overexpression is becoming increasingly important in colorectal and biliary tract cancers. A notable development is the 2024 tumor-agnostic accelerated approval of a HER2-directed antibody-drug conjugate for previously treated, unresectable or metastatic HER2-positive (IHC 3+) solid tumors without a satisfactory alternative — which underscores that the actionable threshold is generally a protein-level (IHC) result, not RNA alone. Still, eligibility, treatment line, and the exact testing required are defined by NCCN's and ESMO's tumor-type-specific guidelines, and some HER2 findings — for example, a low-level copy-number gain or an RNA-only signal — may not be actionable at all.

3My HER2 results don't agree (NGS says amplified but IHC/FISH says negative): what should I do?

Discordant HER2 results are a recognized situation, and the correct next step is usually not to assume one result is wrong, but to resolve them with the correct confirmatory test. NGS measures the gene (DNA copy number or sequence), while IHC directly measures the HER2 protein and FISH directly measures gene copies in the tissue; because they look at different things, they can diverge in a minority of cases. The clinically important point is that approval and guideline thresholds are tied to specific tests: for most indications, HER2 status is defined by IHC, with FISH used to clarify ambiguous protein results, as specified by NCCN and ESMO. So an NGS amplification result that conflicts with a negative IHC, or a HER2 signal based on RNA expression, generally needs to be reconciled by ordering or reviewing the guideline-standard test (IHC and/or FISH) on adequate tumor tissue for your tumor type. A second opinion here is mainly about making sure the correct confirmatory test was performed and interpreted against your tumor type's criteria, because that is what determines whether anti-HER2 therapy is on the table. This is decision support to help you ask the right questions; the testing and treatment decision belongs to your treating oncologist and pathologist.

MSI-H / dMMR and TMB on an NGS report: what do they mean, and when should you get a second opinion?

If your tumor NGS (next-generation sequencing) report says MSI-H (high microsatellite instability), dMMR (mismatch repair deficiency), or TMB-high (high tumor mutational burden), the core message is the same: these are biomarkers that, at guideline level, point toward immunotherapy with immune checkpoint inhibitors. MSI-H/dMMR is one of the few markers that can guide treatment across many cancer types regardless of where the tumor originated, and it was the basis of the first tumor-agnostic immunotherapy approval. Whether this translates into treatment for your specific situation still depends on your tumor type, treatment line, and other findings, and that decision belongs to your treating oncologist. This page explains, in plain language, what these results mean and when a careful second look is worth requesting.

1What does MSI-H or dMMR mean, and is it good or bad?

MSI-H and dMMR describe the same underlying problem in the tumor: the cell's DNA “proofreading” system (mismatch repair) is broken, so the tumor accumulates many small DNA errors, especially in repetitive regions called microsatellites. dMMR usually refers to loss of one or more repair proteins (MLH1, MSH2, MSH6, PMS2) seen on immunohistochemistry, while MSI-H refers to the instability measured by PCR or NGS; in most tumors the two findings agree. From a treatment standpoint, this is generally a favorable finding, because tumors with this defect tend to respond to immune checkpoint inhibitors. The evidence was strong enough that in 2017 the US FDA granted the first tumor-agnostic (tissue/site-independent) accelerated approval of a checkpoint inhibitor for MSI-H/dMMR solid tumors, and a second checkpoint-inhibitor class agent later received a tumor-agnostic approval for dMMR solid tumors as well. An honest caveat: a “favorable biomarker” does not mean every MSI-H tumor will respond, and the strength of the recommendation still depends on your cancer type and where you are in the course of treatment — which is why guidelines (NCCN, ESMO) treat it as one input into the decision, not an automatic prescription. MSI-H/dMMR also carries a second meaning beyond treatment: it can be a clue pointing to Lynch syndrome, an inherited condition, discussed below.

2My report shows MSI-H but TMB is low (or the results disagree). Should I have it re-confirmed?

Yes — an internally inconsistent result is one of the clearest reasons to request a second look before acting on it. MSI-H and high TMB usually track together, because a broken repair system that produces microsatellite instability also tends to generate a high overall mutation count. If a report calls the tumor MSI-H while TMB is low, or the MSI signal is borderline, this discordance raises the possibility of a borderline or false-positive result, and confirmation is reasonable — for example, mismatch repair immunohistochemistry (MMR-IHC), PCR-based MSI testing, or central pathology review. Concordance between methods is generally high, but the rare discordant cases are exactly the ones where a wrong result would change the plan the most. There is also a tumor-biology subtlety your oncologist will weigh: TMB performs more consistently as an immunotherapy signal in cancers with known mutagen exposure (for example, smoking- or UV-related tumors) and less consistently in others, so a TMB number should be read in the context of cancer type, not as a standalone verdict. The practical takeaways: do not treat an unresolved MSI-H/low-TMB combination as settled, and ask whether an orthogonal confirmatory test (such as MMR-IHC) is needed before an immunotherapy decision is finalized.

3Does this come from blood or tissue, and does MSI-H mean I should be tested for Lynch syndrome?

These are two separate but common questions, and both matter for a second opinion. The first concerns sample type: MSI and TMB results obtained from tumor tissue are the validated, guideline-anchored basis for immunotherapy decisions. A blood-based (liquid biopsy) TMB or MSI result depends heavily on how much tumor DNA has shed into the blood, so it can miss findings when circulating tumor fraction is low, and current practice generally treats blood TMB as a surrogate used mainly when tissue is scarce, not a replacement for a tissue result. If an important decision rests only on a blood-based TMB or MSI value, it is reasonable to ask whether tissue confirmation is feasible. The second concerns heredity: an MSI-H/dMMR tumor can be a signal for Lynch syndrome, an inherited cancer predisposition condition, which is why guidelines support considering referral for genetic counseling and germline testing, particularly with a younger age at diagnosis or a suggestive family history. The tumor finding alone does not confirm Lynch syndrome (particularly in colorectal cancer, where additional steps such as MLH1 promoter methylation or BRAF testing help distinguish sporadic from inherited cases), but it is the trigger that starts this conversation — one that carries implications for you and your blood relatives. Your oncologist and a genetics unit can determine the right path.

This content is for educational purposes only. It does not constitute individual medical advice and does not replace the judgment of the treating physician.

• AMP / ASCO / CAP variant interpretation guideline · PubMed
• NCCN Guidelines
• ESMO Guidelines
• OncoKB: EGFR

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