XRCC1 Genotype

Your cells repair damage to their DNA all day long. Some of it comes from ordinary metabolism, and some comes from tobacco smoke, radiation, and certain chemotherapy drugs. This test reads which versions you inherited of XRCC1 (X-ray repair cross-complementing group 1), one of the genes that helps run that repair work.

On its own, your version doesn't predict cancer. The research shows that these variants can change how specific exposures act on your body, and the direction of the effect depends on the exposure. The same variant can look harmful in one setting and protective in another. That is why the details matter.

What the Gene Builds

The gene is on chromosome 19. It makes a protein 633 building blocks long that does no cutting or sealing of its own. It works as a scaffold. It grabs the enzymes that do the repairs and holds them together at the damaged spot.

One end of the protein binds the enzyme that fills in missing DNA letters. The middle binds PARP, an enzyme that detects breaks. The other end binds the enzyme that seals the strand shut. With the protein missing or weakened, those enzymes don't coordinate well.

The job it supports is called base excision repair. It's how a cell cuts out a single damaged DNA letter and puts in a fresh one. The same protein also helps patch single-strand breaks, where one rail of the DNA ladder snaps. Both kinds of damage can come from oxidation. That is chemical wear from reactive oxygen byproducts hitting DNA. Chemicals that stick to DNA can cause it too.

The protein is made in nucleated cells throughout the body. This test doesn't measure how much of it you make. It reads your DNA sequence, which you inherited and which stays the same for life.

The Variants This Test Reads

Most of the research covers a handful of common variants. The names follow a simple rule: Arg399Gln means building block number 399 in the protein switches from arginine to glutamine. The "rs" numbers are catalog labels for the same changes.

• Arg399Gln (rs25487): the most studied change, in the part of the protein that links up with PARP.
• Arg194Trp (rs1799782): a change near the polymerase-binding end, often studied alongside tobacco-related cancers.
• Arg280His (rs25489): a less common change, studied mainly for breast and liver cancer.
• -77T>C (rs3213235): a change in the gene's on-switch region rather than in the protein itself. In laboratory cell experiments, it lowered how strongly the gene turned on.

Full-gene sequencing can also turn up rare changes that single-variant tests miss. Those rare findings usually come back with no established clinical meaning.

What the Variants Do to Repair

The best direct evidence comes from people's own cells. In a study of 524 people, researchers exposed blood cells to DNA-damaging chemicals in the lab and counted chromosome breaks afterward. The number of breaks differed by genotype at codons 194 and 399. So genotype changes how well cells cope with a chemical hit.

The same pattern shows up inside the body. In one human study of aflatoxin exposure, carriers of 399Gln had more toxin stuck to their DNA and more of a blood marker of mutations. Aflatoxin is a mold toxin found on some stored grains and nuts. In 308 healthy adults, 399Gln was also linked to heavier DNA damage marks among nonsmokers.

A separate line of work combined variants from several repair genes, this one included, into a single score. In pooled studies, people with the highest scores had 21.2% higher blood levels of a standard marker of oxidative damage than people with the lowest scores. That result covers the combined score, not this gene alone.

What these findings describe is a dial, not a switch. The common variants shift repair a little in one direction or the other. That's why their effects on disease are modest and why they depend so much on what your cells are exposed to.

Smoking Changes the Picture

Smoking is where genotype matters most clearly, and where the findings look the strangest. Because the effect of the variant depends on the dose, the comparisons below are more useful than any single average.

Sources: Hung et al. (HuGE review of base excision repair genes); Patel et al. (Cancer Prevention Study II Nutrition Cohort).

The same variant pointing in opposite directions for light and heavy smokers looks like a contradiction. It stops looking like one once you drop the idea of a good genotype and a bad genotype. The variant changes how your repair system handles a load of damage, and what that does to cancer risk depends on how big the load is. One proposed explanation is that cells with heavy, unrepaired damage are more likely to die off before they can turn cancerous.

For you, the variant doesn't change the fact that smoking is the risk you can act on. The breast cancer finding doesn't create a new screening plan by itself. It does make tobacco exposure even less worth carrying.

Lung, Head and Neck Cancers

The 194Trp variant has the most consistent protective signal. In a large pooled review, carriers had about 14% lower odds of tobacco-related cancers. The effect is small, and some populations and tumor types have shown the opposite.

For lung cancer in particular, a multicenter study of about 4,400 people found that the common variants played no major independent role. A study from North India looked at five variants and found that a different one, at codon 632, carried the higher lung cancer risk, while 399Gln and 194Trp showed no effect or a protective one.

Head and neck cancer follows a familiar pattern for candidate genes. An early study of 627 people linked the variants to squamous cell cancers of the head and neck, most strongly in the mouth and throat. A later meta-analysis pooling 16,344 people found no meaningful role for 194Trp, 280His, or 399Gln. The larger, later result deserves more weight.

Breast Cancer

Breast cancer has the most research, and it shows the same dependence on exposure. Besides the smoking finding in the table, a study of about 3,900 women found that the 280His variant changed the breast cancer risk linked to smoking. Another study found that XRCC1 variants interacted with smoking-related DNA damage and with fruit and vegetable intake.

Results for the on-switch variant conflict. One study found that women with two copies of -77C had about 2.5 times the odds of breast cancer. A second study of 627 women found no link for that variant alone, though some combinations of variants inherited together were tied to higher risk.

Studies in South Indian and Bangladeshi women linked the 194 and 399 variants to higher breast cancer risk. Risk variants in one population don't always act the same way in another, so where your ancestors came from affects how much these findings apply to you.

Other Cancers

Outside lung and breast, the evidence is mostly single case-control studies. Grouped by what they found:

• Signals of higher risk: a 2025 meta-analysis found a modest prostate cancer signal for 399Gln, clearest in Asian populations. Studies have also tied 194Trp and 280His to hepatitis-related liver cancer in India, 194Trp and 399Gln to glioma in China, and the codon 194 variant to kidney cancer. Glioma is a brain tumor.
• Signals of lower risk: people with at least one copy of 399Gln had a lower risk of the leukemia that can follow earlier cancer treatment. Another study found the 399 variant protective against liver cancer.
• No clear link: a study of about 3,100 people found no association between the 194 or 399 variants and colorectal cancer or polyps. A pooled review found no link between 399Gln and stomach cancer.

Taken together, this gene isn't a strong driver of any single cancer. Its variants show up as small, context-dependent risk modifiers, and the studies that find the biggest effects tend to be the smallest.

Radiation and Chemotherapy Response

Radiation and platinum drugs such as cisplatin, carboplatin, and oxaliplatin kill cancer by damaging DNA. The evidence is not strong enough to choose treatment from this result, but it may add context if you ever face those drugs.

For radiation, a large pooled analysis of women treated for breast cancer found that carriers of one variant (rs2682585) had about 23% lower odds of late skin damage. In head and neck cancer treated with chemotherapy plus radiation, carriers of common variants had more acute radiation side effects, along with a trend toward longer time before the cancer progressed.

With platinum drugs, the evidence splits. In advanced non-small cell lung cancer, the larger pooled analyses, including a meta-analysis of 17 studies, have generally tied 399Gln to poorer treatment response overall, with a more favorable signal only in some Asian subgroups. Some smaller studies point the other way. In one small North Indian study, lung cancer patients with the 399 AA genotype on cisplatin or carboplatin plus docetaxel had roughly three-quarters lower risk of death over follow-up. Studies in Chinese ovarian cancer patients and in bladder cancer patients also linked variant genotypes to better survival.

Other results add to the mix. Some lung cancer cohorts linked 399Gln to shorter survival. Meta-analyses in stomach cancer and in head and neck cancer found the variants didn't predict outcomes.

Where a "weaker" repair variant does track with better response, the logic is simple. Platinum chemotherapy works by damaging DNA, and a tumor that repairs that damage less efficiently holds onto it. The same reduced repair in healthy tissue could explain extra side effects. But the pooled lung cancer data show the direction can't be predicted from the variant alone, and it shifts with ancestry. A variant can raise your cancer risk in one setting and help you in another because it changes the process underneath, not the outcome directly.

Nerve Damage and Other Conditions

A 2026 Egyptian study of 732 people with type 2 diabetes linked the 399 and 194 variants to both a higher risk of diabetic nerve damage and more severe nerve damage. Smaller studies have tied 399Gln to rheumatoid arthritis in a pilot of 100 people and to schizophrenia in South India. These are early leads, not established links.

A One-Time Result

Your genotype was fixed at conception and won't change, so there's nothing to track and no reason to repeat the test. The only exception is a result the lab flags as low-confidence, which should be confirmed by a second method.

The value comes from bringing the result into decisions over the years. It's useful context if you smoke, if you're deciding how seriously to take screening, and above all if you ever face radiation or platinum chemotherapy. No lab value downstream of this gene needs routine monitoring because of your genotype. The markers that matter are the ordinary cancer screening results you should already be keeping up with.

When Results Can Be Misleading

With a genetic result, the errors don't come from what you ate or how you slept. They come from what the assay covered and how the finding gets interpreted.

• Coverage gaps: a result only covers the variants and gene regions the lab reports. A normal result on another panel, such as a medication-response panel, doesn't mean this gene was checked.
• Ancestry: how common each variant is, and how strongly it affects risk, varies between populations. A finding from Asian or South Asian cohorts may not transfer to you, and the reverse is also true.
• Uncertain variants: rare changes often come back as variants of uncertain significance. Laboratories applying the same rules disagree on classification roughly 10% to 40% of the time across genetic testing broadly.
• Calling errors and source: low-confidence sequencing calls may need confirmation by Sanger sequencing. Tumor tissue should not be used to infer your inherited result, and consumer DNA reports may use different variant sets and quality controls.

What to Do With Your Result

If you carry only the common variants, don't change your screening intervals because of them. No study has shown that testing healthy people for these variants catches cancer earlier or improves outcomes, and there are no validated steps tied to carrying them. Use the result as context. Keep up with age-appropriate cancer screening, and if you smoke, quitting is still the lever that counts.

If you have a family history of cancer, this gene isn't the right place to look for an answer. Order a full hereditary cancer gene panel, which covers the high-impact genes that actually change screening plans, and consider a genetic counselor to help you read it.

If you're facing radiation or platinum chemotherapy, give the result to your oncologist and radiation oncologist. Because the evidence is mixed, it won't decide your treatment on its own. It can shape how closely your team watches for skin and tissue side effects.

If the report lists a rare or uncertain variant, check whether it was confirmed by Sanger sequencing, and check whether the lab offers reanalysis as evidence changes. A genetic counselor is worth seeing at that point. Your biological relatives each share roughly half your variants, but for common variants with small effects, their own family history is a better reason to test than your result.