KRAS G12C is a specific genetic mutation found in cancer cells where a single amino acid swap in the KRAS protein causes it to get stuck in an “on” position, driving uncontrolled cell growth. It occurs most frequently in non-small cell lung cancer (NSCLC), where about 7.4% of all cases carry this mutation, and in colorectal cancer, where it appears in roughly 3.4% of cases. What makes KRAS G12C significant is that it was the first KRAS mutation to become treatable with targeted drugs, after decades of being considered “undruggable.”
How the Mutation Works
The KRAS protein normally acts like a molecular switch. It flips on to signal cells to grow and divide, then flips back off when the job is done. In cells with a G12C mutation, the amino acid glycine at position 12 on the protein is replaced by cysteine. That single substitution traps the protein in its active state, so the growth signal never shuts off. Cells multiply uncontrollably, and a tumor forms.
The “G12C” name is shorthand: G stands for glycine (the original amino acid), 12 is the position on the protein, and C stands for cysteine (the replacement). Other KRAS mutations follow the same naming pattern, like G12D or G12V, each swapping in a different amino acid. G12C is the most common KRAS mutation in lung cancer and one of the few that currently has FDA-approved targeted treatments.
How KRAS G12C Is Detected
Finding a KRAS G12C mutation requires molecular testing of the tumor. The standard approach is next-generation sequencing (NGS), which scans tumor DNA for hundreds of known cancer-driving mutations at once. This can be done on a tissue sample from a biopsy or on a blood draw called a liquid biopsy, which captures fragments of tumor DNA circulating in the bloodstream.
Liquid biopsy is especially useful when a tissue sample isn’t available or isn’t large enough for testing, which is common in advanced lung cancer. In one clinical practice review of 194 liquid biopsies, NGS detected KRAS mutations in about 19% of samples, and G12C was the most common subtype found, accounting for 36% of those KRAS-positive cases. The FDA has approved specific companion diagnostic tests for identifying G12C, including both tissue-based and blood-based options, to determine whether a patient qualifies for targeted therapy.
Which Cancers Carry This Mutation
KRAS G12C appears across several cancer types but is most clinically relevant in two. In non-small cell lung cancer, about 21% of tumors have some form of KRAS mutation, and G12C is the single most common subtype at 7.4% of all NSCLC cases. It’s especially prevalent in people with a history of smoking. In colorectal cancer, KRAS mutations are also common, but G12C specifically accounts for about 3.4% of cases. Pancreatic cancer frequently harbors KRAS mutations too, though G12C is less common there than other subtypes like G12D.
Targeted Drugs That Block G12C
For decades, the KRAS protein was considered undruggable because its surface lacked an obvious place for a drug to latch onto. That changed when researchers discovered a hidden pocket on the protein, called the Switch II pocket, that only appears when the mutant cysteine is present. Drugs designed for KRAS G12C form a permanent (covalent) bond with that cysteine, locking the protein in its inactive state and shutting down the growth signal.
Sotorasib became the first KRAS-targeted drug to receive FDA approval in May 2021, cleared for adults with locally advanced or metastatic NSCLC carrying a confirmed G12C mutation who had already received at least one prior treatment. In the pivotal CodeBreaK 100 trial, sotorasib produced a tumor response in 41% of patients. The median duration of that response was 12.3 months, median progression-free survival was 6.3 months, and median overall survival reached 12.5 months, with about one-third of patients alive at two years.
Adagrasib, a second G12C inhibitor, followed with its own approval for a similar patient population. Both drugs are taken as daily pills, and their side effects tend to be more manageable than traditional chemotherapy, though liver enzyme elevations, diarrhea, and fatigue are among the more common issues reported.
How G12C Affects Immunotherapy Response
KRAS mutation subtype can influence how well immunotherapy works. In lung cancer patients with high levels of a protein called PD-L1 (50% or higher), those with G12C mutations had survival outcomes similar to patients with no KRAS mutation at all, with a median overall survival of about 19.9 months. That’s notably better than the G12V subtype, which was linked to significantly worse survival of just 8.2 months in the same group.
The picture is less clear for patients with lower PD-L1 levels. When PD-L1 was below 1%, G12C patients had a median survival of 9.4 months compared to 12.4 months for those without any KRAS mutation. So while G12C doesn’t appear to make immunotherapy less effective for patients with high PD-L1 expression, it may carry a slight disadvantage when PD-L1 is low.
Why Tumors Eventually Resist Treatment
Most patients who respond to G12C inhibitors eventually develop resistance. A study published in the New England Journal of Medicine found detectable resistance mechanisms in 45% of patients analyzed, and nearly one in five had multiple resistance mechanisms happening simultaneously.
Resistance takes several forms. Some tumors acquire new mutations directly in the KRAS gene itself, including changes at the drug-binding site that prevent the inhibitor from attaching. In one case, the original G12C mutation picked up an additional change that converted it to G12W, completely blocking drug binding. Other tumors develop secondary KRAS mutations on a separate copy of the gene, such as G12D, G12V, or G13D, essentially activating a backup growth signal the drug can’t touch.
A third category of resistance involves bypass pathways, where the tumor activates entirely different growth-promoting genes. These include amplification of a gene called MET, activating mutations in genes like NRAS and BRAF, and oncogenic gene fusions. In two patients, the tumor physically transformed from one cell type (adenocarcinoma) to another (squamous cell carcinoma) without any identifiable genetic resistance mechanism, suggesting the cancer can sometimes sidestep the drug through structural changes alone.
Combination Strategies to Overcome Resistance
Because resistance is common with single-drug treatment, researchers are testing G12C inhibitors alongside other therapies. Combinations under investigation pair G12C inhibitors with drugs that block proteins upstream or downstream in the same signaling chain, including inhibitors targeting SHP2, SOS1, MEK, and the cell-cycle regulator CDK4/6. The goal is to cut off the escape routes tumors use when the primary pathway is blocked.
Combinations with immunotherapy are also being studied. Since G12C-mutant lung cancers often respond to immune checkpoint inhibitors on their own, adding a targeted G12C drug could potentially deepen and extend that response. Early results from these combination trials are being reported, though none have yet changed the standard treatment approach. For now, G12C inhibitors are typically used after a patient has already tried chemotherapy or immunotherapy, with combination regimens likely to reshape treatment sequencing in the coming years.