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What is the most common EGFR mutation?

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The most common EGFR mutations in lung cancer are Exon 19 deletions and Exon 21 L858R, making up 80-90% of cases. These classic mutations respond exceptionally well to daily oral medications called TKIs. Identifying your specific mutation helps your doctor choose the most effective targeted therapy.

Key Takeaways

  • • An EGFR mutation causes lung cancer cells to grow out of control, but it also creates a target for highly effective specialized treatments.
  • • Exon 19 deletions and Exon 21 L858R are the most common EGFR mutations, accounting for up to 90% of cases.
  • • Classic EGFR mutations are highly sensitive to daily targeted oral medications called Tyrosine Kinase Inhibitors (TKIs).
  • • If the cancer adapts to older treatments by developing a T790M resistance mutation, newer third-generation TKIs can be used to regain control.
  • • Uncommon mutations, such as Exon 20 insertions, have their own specific targeted therapies available, including new IV medications.

When doctors test a lung cancer biopsy for biomarkers, finding an EGFR mutation (Epidermal Growth Factor Receptor mutation) is a critical step in determining the best treatment plan. If you are asking about the most common EGFR mutation, the answer involves two specific genetic changes that together make up the vast majority of cases.

Before looking at the specific types, it helps to understand what EGFR is. Normally, EGFR is a protein that acts like an “on/off” switch to help cells grow and divide when they should. However, an EGFR mutation means there is an error in the cancer cell’s DNA that leaves this switch permanently stuck in the “on” position, causing the cells to multiply out of control [1].

When you read a pathology report, you will often see the word “Exon” followed by a number. Think of an Exon as a specific chapter in the gene’s instruction manual. By identifying exactly which “chapter” has the error, doctors can choose the most effective medicine for you.

The single most common EGFR mutation is the Exon 19 deletion, which accounts for roughly 45% of all EGFR mutations in non-small cell lung cancer (NSCLC) [2][3]. Close behind it is the Exon 21 L858R point mutation, which makes up about 38% to 40% of cases [2][3]. Together, these two are often called “classic” or “sensitizing” mutations, and they account for 80% to 90% of all EGFR mutations in lung cancer [1][3][4].

The “Classic” Mutations: Exon 19 Deletion and L858R

These two most common mutations are called “sensitizing” because they make the cancer highly sensitive to a class of targeted therapies called EGFR Tyrosine Kinase Inhibitors (TKIs) [1][5]. These are typically daily oral medications (pills you take at home), which for many patients replaces the need for traditional IV chemotherapy.

While both of these mutations respond very well to TKIs, there are slight differences:

  • Exon 19 Deletions (Ex19del): This mutation happens when a small piece of DNA is missing from the EGFR gene. Studies have shown that patients with this mutation often have particularly strong responses to targeted therapies, allowing them to live longer while keeping the cancer under control (a clinical term known as progression-free survival) [6][4][7].
  • Exon 21 L858R: This is a point mutation, meaning just a single letter in the DNA code has been swapped. Patients with this mutation also respond very well to TKIs, though their tumors might behave slightly differently than those with an Exon 19 deletion [7][4].

Today, the standard first-line treatment for both of these classic mutations is often a highly advanced, third-generation TKI (such as osimertinib) [8][9]. While these oral therapies are highly effective, they can cause side effects—most commonly skin rashes and diarrhea—that you should discuss with your care team so they can help you manage them.

The Third Most Common: Exon 20 Insertions

Making up about 4% to 10% of EGFR mutations, the Exon 20 insertion is the third most common type [10][11]. Unlike the classic mutations, Exon 20 insertions typically do not respond well to older TKIs [10][11]. Because of this, they are treated differently. New specialized targeted therapies, such as the IV medication amivantamab, are now available specifically to treat this exact mutation, and many promising clinical trials are ongoing [11][12].

“Uncommon” Mutations

The remaining 10% to 20% of EGFR mutations are classified as “uncommon” [1][13]. These include point mutations known as G719X, L861Q, and S768I [1][14]. Some patients even have “complex” mutations, meaning they have more than one EGFR mutation at the same time [15][16]. Certain targeted therapies (like afatinib or osimertinib) have shown strong effectiveness against these uncommon types [17][18][19].

The Resistance Mutation: T790M

If a patient is treated with older (first- or second-generation) TKIs, over time, the cancer can adapt to the medication. When this happens, it is called acquired resistance. The most common way the cancer does this is by developing a new EGFR mutation called T790M [20][21]. This new mutation acts like a shield, blocking the older drugs from working [22].

The T790M mutation occurs in up to 60% of cases where the cancer has adapted to older TKI treatments [20][23]. While this can sound intimidating, there are already highly effective treatments designed precisely for this situation. If a T790M mutation is detected (often through a simple blood test called a liquid biopsy) [24], doctors typically switch the treatment to a third-generation TKI (like osimertinib), which was specifically created to bypass this “shield” and regain control of the cancer [8][9]. Because doctors know this resistance can happen, they now frequently prescribe third-generation TKIs right from the start to prevent the T790M mutation from ever developing.

Frequently Asked Questions

What is the most common EGFR mutation in lung cancer?
The most common type is the Exon 19 deletion, which accounts for about 45% of all EGFR mutations. The second most common is the Exon 21 L858R point mutation, making up nearly 40% of cases. Together, these are often called 'classic' or 'sensitizing' mutations.
How are classic EGFR mutations like Exon 19 and L858R treated?
These classic mutations are highly sensitive to targeted therapies called Tyrosine Kinase Inhibitors (TKIs). These are typically daily oral pills that you take at home, which for many patients replaces the need for traditional IV chemotherapy.
What does a T790M mutation mean on my pathology report?
T790M is an acquired resistance mutation that can develop if your cancer adapts to older TKI medications. If this happens, doctors can usually switch your treatment to a newer, third-generation TKI designed specifically to bypass this resistance and regain control of the cancer.
Are there treatments for uncommon EGFR mutations?
Yes. While mutations like Exon 20 insertions or point mutations like G719X are less common, there are specific targeted therapies and clinical trials available. Your oncologist will tailor your medication to the exact mutation found in your biopsy.
What are the common side effects of targeted therapy for EGFR mutations?
Like many medications, targeted therapies can cause side effects. The most common side effects of TKIs are skin rashes and diarrhea. You should discuss these with your care team so they can help you manage them effectively.

Questions for Your Doctor

  • • What specific EGFR mutation did my biopsy show (e.g., Exon 19 deletion, Exon 21 L858R, or an uncommon mutation)?
  • • Am I eligible to be treated with a targeted therapy (TKI), and will it be a daily pill or an IV infusion?
  • • Which generation of TKI are you recommending for my first treatment, and why?
  • • What side effects should I expect from this medication, and what should I do if I experience a skin rash or diarrhea?
  • • How frequently will we do scans or liquid biopsies to ensure the medication is still working and the cancer hasn't adapted?

Questions for You

  • • What were my initial symptoms that led to this diagnosis, and how have they changed recently?
  • • Have any blood relatives been diagnosed with lung cancer or known genetic mutations?
  • • What are my personal care priorities when balancing treatment effectiveness with potential side effects like skin rashes or fatigue?
  • • How comfortable am I with the idea of taking a daily pill at home versus receiving treatments in a clinic setting?

Want personalized information?

Type your question below to get evidence-based answers tailored to your situation.

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This guide explains EGFR mutation types for educational purposes. Your oncologist and care team are the best sources for interpreting your specific biopsy results and determining your treatment plan.

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