MLH1 Gene: Lynch Syndrome, Cancer Risks, and What to Do
Medically reviewed by LifeShield Medical Team, Board-Certified Genetic Counselors & Clinical Advisors
Last updated February 20, 2026
Key Takeaways
- MLH1 is a mismatch repair gene that corrects errors occurring during DNA replication. Mutations cause Lynch syndrome, the most common hereditary cancer predisposition.
- MLH1 mutation carriers face a 40–80% lifetime risk of colorectal cancer and women face a 25–60% lifetime risk of endometrial cancer.
- Lynch syndrome also increases risk for ovarian, gastric, urinary tract, and other cancers.
- Enhanced surveillance with colonoscopy every 1–2 years starting at age 20–25 is strongly recommended and has been shown to reduce cancer mortality.
- Each child of an MLH1 carrier has a 50% chance of inheriting the mutation, making cascade testing essential for family members.
MLH1 In Plain English
Every time your cells divide, they need to copy all of your DNA. This copying process is fast, and small errors, called mismatches, inevitably occur. The MLH1 gene makes a protein that is part of your body’s spell-check system for DNA. It scans newly copied DNA, finds these mismatches, and coordinates their correction. When MLH1 is mutated, this proofreading system fails, and errors accumulate with each cell division. Over time, this leads to a condition known as Lynch syndrome, which significantly increases the risk of several types of cancer.
Lynch syndrome (previously called hereditary nonpolyposis colorectal cancer, or HNPCC) is the most common hereditary cancer predisposition syndrome, affecting an estimated 1 in 279 people. Despite its prevalence, most people with Lynch syndrome are undiagnosed. For a comprehensive overview, see our guide to Lynch syndrome. The MSH2 gene is MLH1's partner in the mismatch repair process, and mutations in either gene account for the majority of Lynch syndrome cases. To understand how Lynch syndrome fits within the broader landscape of inherited cancer risk, see our guide on hereditary cancer screening.
If you receive a positive MLH1 result, see our guide on what to do after a positive genetic test result. For help interpreting the different types of results, read our resource on understanding genetic test results. If your result is a variant of uncertain significance, see our VUS guide for guidance on what that means. Because Lynch syndrome is highly actionable and each child of a carrier has a 50% chance of inheriting the mutation, cascade testing for family members is one of the most impactful steps you can take. For guidance on navigating the counseling process, see what to expect from genetic counseling.
Cancer Risks Associated with MLH1 Mutations
| Cancer Type | Lifetime Risk | General Population | Source |
|---|---|---|---|
| Colorectal cancer | 40–80% | 4.1% | NCCN v2.2025; Dominguez-Valentin et al., Genet Med 2020 |
| Endometrial cancer (women) | 25–60% | 3.1% | NCCN v2.2025; Dominguez-Valentin et al., Genet Med 2020 |
| Ovarian cancer | 4–20% | 1.2% | NCCN v2.2025 |
| Gastric cancer | 1–13% | 0.8% | NCCN v2.2025 |
| Urinary tract cancer | 1–18% | < 1% | NCCN v2.2025 |
Who Should Consider MLH1 Testing?
NCCN recommends MLH1 testing for individuals whose tumors show loss of MLH1 protein expression or microsatellite instability, individuals meeting Amsterdam II or revised Bethesda criteria, those with a personal or family history of multiple Lynch-associated cancers, or when a known MLH1 mutation exists in the family.
Should You Get Tested?
Free 60-second screener based on NCCN guidelines — no account needed
Check Your Eligibility →Questions to Ask Your Genetic Counselor About MLH1
- What is my specific MLH1 mutation, and how does it affect my cancer risk profile?
- How early should I start colonoscopy screening, and how often should I repeat it?
- Should I consider daily aspirin therapy to reduce my colorectal cancer risk?
- As a woman, when should I consider risk-reducing hysterectomy?
- Which of my family members should be tested for this MLH1 mutation?
- Are there screening recommendations for gastric or urinary tract cancers?
- Is immunotherapy effective for MLH1-associated cancers?
What a MLH1 Result Means for Your Family
MLH1 mutations follow autosomal dominant inheritance. Each first-degree relative (parent, sibling, child) has a 50% chance of carrying the mutation. Because Lynch syndrome is common yet underdiagnosed, cascade testing is one of the most impactful public health interventions available. Identifying carriers allows them to begin colonoscopy surveillance, which has been proven to reduce colorectal cancer mortality. NCCN strongly recommends offering testing to all at-risk blood relatives.
Inheritance Pattern: Autosomal dominant
Prevalence: Lynch syndrome affects approximately 1 in 279 individuals; MLH1 accounts for approximately 30–35% of Lynch syndrome cases
Management Options If Positive:
- Colonoscopy every 1–2 years starting at age 20–25
- Consider daily aspirin (discuss with physician)
- Annual endometrial sampling starting at age 30–35 for women
- Risk-reducing hysterectomy and oophorectomy after childbearing (women)
- Upper endoscopy every 2–4 years starting at age 30–40
- Annual urinalysis starting at age 25–30
Medical Disclaimer: This content is for informational purposes only and does not constitute medical advice. It is based on published NCCN clinical guidelines and peer-reviewed research. Always consult a qualified healthcare provider for medical decisions. LifeShield content is reviewed by board-certified genetic counselors.