If you have ever read a genetic or tumour-testing report and stumbled over the words germline and somatic, you are in good company. The two terms sit at the heart of modern cancer genetics, yet they describe very different things — one a variant you were born with, the other a change a tumour picked up along the way. This explainer walks through what each term means, how laboratories tell them apart, and why the difference matters for you and for your relatives. It is background reading, not a substitute for advice about your own results.
The core distinction
A germline variant is present in the egg or sperm and therefore in the DNA of essentially every cell in the body. Because it is in the reproductive cells, it can be passed from parent to child — it is hereditary. The U.S. National Cancer Institute frames germline testing as a way to identify variants tied to an inherited cancer risk so that risk-management decisions can be planned.
A somatic variant, by contrast, arises before or during tumour development. It is not in the egg or sperm, so it is generally confined to the tumour or the tissue where the cancer grew. It is not inherited and cannot be passed to children. Somatic changes are what tumour (or “molecular profiling”) tests look for to understand what is driving a particular cancer.
Two different samples, two different questions
The sample type usually signals which question is being asked:
- Germline testing typically uses blood or a saliva/buccal sample, because the variant is everywhere in the body. It asks: was this person born with an increased risk?
- Somatic testing uses tumour tissue from a biopsy or surgery — or a liquid biopsy capturing tumour DNA in blood. It asks: what changes are driving this tumour, and do they point to a treatment?
| Feature | Germline variant | Somatic variant |
|---|---|---|
| Where it lives | Every cell in the body | Tumour / affected tissue only |
| Inherited? | Yes — can pass to children | No |
| Usual sample | Blood or saliva | Tumour tissue or liquid biopsy |
| Main question | Inherited cancer risk | Tumour drivers and treatment |
| Relatives affected? | Possibly | No |
How labs tell them apart in tumour data
When only tumour tissue is sequenced (“tumour-only” testing), separating the two is genuinely tricky. One clue is the variant allele fraction (VAF) — roughly, the proportion of sequencing reads carrying the change. A heterozygous germline variant tends to sit near 50% across both tumour and normal cells, while a somatic change scales with how much of the sample is actually tumour. The signal is imperfect: one analysis found that about 22% of germline variants measured between 40–60% in normal tissue fell outside that window in the paired tumour. To sharpen the call, labs use high sequencing depth — often greater than 500× — alongside copy-number and tumour-purity models. The cleanest approach remains a matched normal sample, which is why a suspected inherited finding is usually confirmed on a separate germline test.
When tumour tests reveal something hereditary
Tumour panels sometimes surface a change that looks inherited — an incidental germline finding. In one review of 6,060 sequencing tests, suspicious variants in genes such as TP53, ATM, CHEK2, BRCA1 and APC turned out, after follow-up, to be mostly clonal haematopoiesis or mosaicism rather than true germline events — only a minority were confirmed hereditary. That is exactly why guideline bodies such as NCCN and ASCO treat dedicated germline testing, with confirmatory follow-up, as the proper way to establish an inherited result.
Why the labels are not a verdict
Finding a variant is only half the story; the other half is classification. Under the ACMG/AMP framework, variants are sorted into five tiers — Pathogenic, Likely pathogenic, Variant of Uncertain Significance (VUS), Likely benign, and Benign — by weighing population, computational, functional and segregation evidence. “Likely” tiers carry a greater-than-90% probability either way; a VUS simply means the evidence is currently insufficient or conflicting. A VUS is not a diagnosis and should not, on its own, drive medical decisions.
Where the variant is inherited and pathogenic, the stakes can be high. Germline pathogenic variants in BRCA1 and BRCA2 carry an estimated lifetime breast-cancer risk of roughly 72% and 69% respectively by older age, against about 12% for women in the general U.S. population — a reminder of why an inherited finding ripples out to relatives who may wish to be tested.
This focus on careful, citation-backed interpretation echoes the heritage of hereditary-cancer registries such as PROMPT, whose work has long appeared in the oncology literature.
The bottom line
Germline variants are inherited and present body-wide; somatic variants are acquired and confined to a tumour. The first speaks to risk shared with family; the second to what is driving a specific cancer. Sample type, allele fractions and confirmatory testing are how laboratories keep the two straight — and a tier label, especially a VUS, is a starting point for discussion, not a conclusion.
Educational explainer, not medical advice. For your own testing or results, speak with a clinician or genetic counsellor.