The question "how many cancer risk genes are there?" has a complicated answer. The number depends on what you count and why.

The Full Landscape

Researchers have identified hundreds of genes in which variants have been associated with cancer risk in at least some studies. The Cancer Gene Census maintained by the Wellcome Sanger Institute lists over 700 genes with evidence of involvement in cancer.

However, most of these associations come from studies of somatic mutations in cancer cells — changes that arise in tumors during a person's lifetime, not inherited variants passed from parent to child.

Clinically Actionable Hereditary Cancer Genes

For hereditary cancer risk — the kind assessed by germline genetic testing — the number of well-established, clinically actionable genes is considerably smaller.

The most widely used clinical guidelines, including those from the National Comprehensive Cancer Network (NCCN), focus on genes where:

1. The association with cancer risk is well established in multiple independent studies
2. The magnitude of risk increase is clinically meaningful
3. Evidence-based management recommendations exist — meaning there are specific screening, prevention, or treatment options that can reduce risk or improve outcomes

By these criteria, the number of genes with strong clinical evidence and actionable management guidelines is roughly 25 to 50, depending on the cancer type and the specific guideline.

The Most Established Genes

The genes with the strongest evidence and most developed clinical guidelines include:

Breast and ovarian cancer: BRCA1, BRCA2, PALB2, ATM, CHEK2, RAD51C, RAD51D, BRIP1

Colorectal and Lynch syndrome: MLH1, MSH2, MSH6, PMS2, EPCAM, APC, MUTYH

Other hereditary cancers: TP53 (Li-Fraumeni), CDH1 (gastric/lobular breast), STK11 (Peutz-Jeghers), PTEN (Cowden syndrome), VHL, RET, SDHA/B/C/D (paraganglioma/pheochromocytoma), RB1 (retinoblastoma), WT1 (Wilms tumor)

Pancreatic cancer: BRCA1, BRCA2, PALB2, ATM, MLH1, MSH2, MSH6, PMS2, CDKN2A

Prostate cancer: BRCA1, BRCA2, ATM, CHEK2, HOXB13

Genes of Uncertain Significance at the Population Level

Beyond the well-established genes, there is a large and growing list of genes where some evidence of cancer association exists but clinical management guidelines are less clear. These include genes where:

• The magnitude of risk increase is modest or uncertain
• Evidence comes from fewer or smaller studies
• No consensus management recommendations have been established

Finding a pathogenic variant in one of these genes can be genuinely uncertain — it may represent a real risk factor, but the appropriate clinical response is not yet well defined.

Why the Number Matters

Larger multi-gene panels test more genes, which increases the chance of finding a variant of uncertain significance (VUS). A VUS is not the same as a pathogenic variant — it means there is not yet enough evidence to classify the variant as harmful or benign.

At Genetic Insights, we help patients understand not just what genes were tested, but what the evidence says about each finding — and what that means for clinical decision-making.

Key Takeaway: Hundreds of genes have some cancer association, but roughly 25–50 have strong clinical evidence and actionable management guidelines. The distinction between well-established genes and genes of uncertain clinical significance matters enormously for interpreting results.

Sources: NCCN Guidelines — Genetic/Familial High-Risk Assessment; Wellcome Sanger Cancer Gene Census