An oncogene is a mutated version of a normal gene that drives cells to grow and divide uncontrollably, potentially leading to cancer. Every person carries the original, healthy versions of these genes, called proto-oncogenes, which play essential roles in normal cell growth. When a proto-oncogene is damaged by mutation, fused with another gene, or copied too many times, it becomes permanently switched on and can push a cell toward cancerous behavior.
Proto-Oncogenes: The Normal Version
Proto-oncogenes are part of your normal genetic toolkit. They produce proteins that tell cells when to grow, divide, or stay alive. Think of a proto-oncogene as a gas pedal on a car: it helps the cell move forward through its life cycle at the right speed. These genes are tightly regulated, turning on only when the body sends the right signals, like during wound healing or tissue development.
The problem starts when something damages a proto-oncogene in a way that removes those built-in controls. The gas pedal gets stuck down. The gene, now an oncogene, produces a protein that is either always active, overproduced, or structurally altered so it no longer responds to the body’s “stop” signals. A single mutated copy of the gene is often enough to cause problems, which is a critical distinction from other cancer-related genes.
How Proto-Oncogenes Become Oncogenes
Three main genetic events can flip the switch from proto-oncogene to oncogene: point mutations, gene amplification, and chromosomal rearrangements. Each one either changes the structure of the protein the gene produces or floods the cell with too much of it.
Point Mutations
A point mutation is a single-letter change in the DNA code that alters one amino acid in the resulting protein. This tiny change can land in a critical regulatory region, leaving the protein permanently active. The RAS family of genes is the most common example. RAS mutations appear in an estimated 15% to 20% of all human tumors, with especially high rates in specific cancers: roughly 90% of pancreatic cancers, 50% of colon cancers, and 30% of lung adenocarcinomas carry a mutated version of K-RAS. Most of these mutations hit a single spot in the gene (codon 12), which is enough to lock the RAS protein into its “on” position.
Gene Amplification
Instead of changing the gene’s structure, amplification produces extra copies of it, sometimes hundreds. More copies mean more protein, which overwhelms the cell’s normal regulatory machinery. About 20% to 30% of breast and ovarian cancers show amplification of the MYC gene. HER2, a growth factor receptor gene, is amplified in roughly 15% to 30% of breast and ovarian cancers. In glioblastomas (an aggressive brain cancer), the gene for the epidermal growth factor receptor is amplified in up to 50% of cases.
Chromosomal Rearrangements
Sometimes large pieces of chromosomes break off and reattach in the wrong place, fusing parts of two different genes together. The resulting hybrid protein often has an enzyme domain that is always active because it’s been disconnected from the regulatory portions that would normally keep it in check. This type of rearrangement is particularly well known in certain blood cancers and thyroid cancers.
What Oncogenes Actually Do Inside Cells
Oncogene proteins don’t all do the same thing, but they converge on a common outcome: pushing the cell to divide when it shouldn’t. Some oncogene proteins sit on the cell surface and mimic growth signals. Others work inside the cell as relay switches in signaling chains. Still others enter the nucleus and directly control which genes get turned on or off.
MYC is one of the most studied oncogenes and illustrates how far-reaching the effects can be. When overactive, the MYC protein ramps up the production of molecules that push cells through their division cycle while simultaneously suppressing the proteins that act as brakes. It boosts the cell’s energy metabolism, protein production, and the raw materials needed to build new DNA. Essentially, MYC rewires the entire cell to prioritize growth and replication above everything else.
HER2 works differently. It’s a receptor protein on the cell surface that normally helps relay growth signals from outside the cell. What makes HER2 unusual is that it has no natural “off switch” the way related receptors do. In healthy cells, HER2 is produced at low levels, so this isn’t a problem. But when the gene is amplified and the cell is flooded with HER2 protein, the sheer quantity of receptors triggers constant growth signaling with no external signal required.
Oncogenes vs. Tumor Suppressor Genes
Cancer typically involves damage to two types of genes, and understanding how they differ matters. Oncogenes are gain-of-function mutations: the gene acquires a new, harmful ability. A single mutated copy is usually enough to contribute to cancer because the overactive protein dominates the cell’s behavior. Tumor suppressor genes are the opposite. They normally slow cell division or trigger damaged cells to self-destruct. Cancer requires a loss-of-function mutation, and typically both copies of the gene must be knocked out before the protective effect is lost.
Using the car analogy: an oncogene is a gas pedal stuck to the floor, while a broken tumor suppressor gene is a failed brake. Most cancers involve both types of damage, not just one, which is part of why cancer usually develops over years through an accumulation of multiple genetic hits.
How Oncogene Testing Works
Identifying which oncogenes are active in a tumor is now a routine part of cancer diagnosis for many cancer types. The most direct approach is testing a tissue sample (biopsy) for specific mutations or gene amplifications using sequencing technology. When a tissue biopsy isn’t safe or feasible, a blood draw called a liquid biopsy can detect fragments of tumor DNA circulating in the bloodstream. This approach is clinically accepted when the results will directly guide treatment decisions and there’s an FDA-approved therapy linked to the specific genetic finding.
HER2 testing in breast cancer is one of the most familiar examples. Pathologists check the tumor tissue for excess HER2 protein or extra copies of the HER2 gene. The result determines whether a patient is eligible for drugs that specifically target the HER2 protein. Similarly, lung cancer patients are routinely tested for mutations in genes like EGFR and rearrangements involving ALK, because targeted treatments exist for each.
Targeted Therapies That Block Oncogene Proteins
The identification of specific oncogenes has transformed cancer treatment. Rather than relying solely on chemotherapy, which kills dividing cells indiscriminately, targeted therapies are designed to block the exact protein an oncogene produces. The FDA has approved dozens of these drugs, and they fall into two broad categories.
Small-molecule inhibitors are pills that enter cells and physically block the active site of an oncogene protein, preventing it from sending growth signals. Imatinib (Gleevec) was one of the first, turning chronic myeloid leukemia from a near-certain death sentence into a manageable condition by blocking the product of a specific chromosomal rearrangement. Osimertinib (Tagrisso) targets mutated EGFR in lung cancer. Sotorasib (Lumakras) was the first drug to successfully target a specific KRAS mutation, a protein once considered “undruggable” for decades.
Monoclonal antibodies are lab-made proteins delivered by infusion that attach to oncogene products on the cell surface. Trastuzumab (Herceptin) binds to the HER2 receptor and has dramatically improved survival in HER2-positive breast cancer. Cetuximab (Erbitux) targets the EGF receptor in colorectal and head and neck cancers.
These treatments work best when the specific oncogene driving a patient’s cancer has been identified through genetic testing, which is why oncogene testing and targeted therapy go hand in hand. Not every cancer has a known targetable oncogene, and tumors can develop resistance to targeted drugs over time, but the growing list of approved therapies continues to expand the number of patients who benefit from this approach.