- Carries a changed copy of PRNP
- Does not carry it
- 50%chance for each child
Each child of a parent with the change has a 50 percent chance of inheriting it. The drawing shows two of four children for simplicity; each chance is separate, so a real family can differ. Whether someone who inherits it becomes ill depends on the change. A genetic counsellor can explain what applies in a family. Each adult decides for themselves whether to find out.
Most prion disease has nothing to do with inheritance. Between about 5 and 15 in every 100 cases are the genetic kind, caused by a change in PRNP, the gene that carries the instructions for prion protein. The rest happen with no known cause, or after contact with prions during medical care many years ago.
What does it mean that it is genetic?
The change sits in one of the two copies of the gene a person carries. One altered copy is enough for the illness to be possible, which is why these conditions pass down a family rather than skipping generations. Each child of a person who carries the change may inherit it, and relatives are usually told that the chance for each child is one in two.
That figure is being looked at again. A 2025 study of 24 families in Spain found the change was inherited more often than one in two for two particular variants, around two thirds of the children in each group. The researchers do not yet know why, and they say counselling figures may need revising. A genetic counsellor can explain what the current numbers are for a particular family, which is work no web page can do.
Who do I talk to first?
A genetic counsellor, before any test. Genetic counselling comes first explains what happens in those appointments, and finding a counsellor covers how to reach one. The usual route is a referral from the GP or the neurologist.
Having a test is a personal choice. Deciding not to know is a complete and valid answer, and a counsellor should support it as readily as the opposite. The right not to know is about that choice.
What is on the other pages here?
Inherited prion disease explained describes the conditions themselves. Predictive testing sets out what a test involves for someone with no symptoms, and what a result can tell you covers what it does and does not settle.
For the practical side, there is talking with family, insurance and work, family planning options and joining research without testing. If research is on your mind, how trials work explains the groundwork.
What should the family know meanwhile?
There is no evidence that CJD passes from person to person through ordinary social contact or ordinary care. Hugging, sharing meals and nursing someone at home are safe.
Two practical points do follow a family history. People with prion disease, and relatives at increased risk because of family or medical history, cannot give blood or organs. It is also worth telling a care team about the family history before surgery or dental work, so they can use the precautions they have for certain procedures.
Nothing here can tell you what applies to one person. That belongs with a genetic counsellor or the treating team. Understand and Care is the hub for the rest of this section, and the diseases explains the illnesses in plain words.
Explore
- Genetic counselling comes firstWhy a conversation with a genetic counsellor comes before any genetic test in a family with prion disease, and what those appointments involve.
- Family planning optionsFamily planning in genetic prion disease: what direct and exclusion PGT are, prenatal testing, the published prion disease experience, and who decides what.
- Inherited prion disease explainedWhat inherited prion disease is, which conditions belong to it, how it passes down a family, and who can answer questions about your own family.
- Insurance, work and genetic informationWhat the rules say about genetic test results, insurance and employment, and why anyone weighing a test should ask about this before testing, not after.
- Joining research without learning your statusYou can take part in some prion disease research without being told whether you carry a family gene change. How that has been done, and what to ask.
- Predictive testing: what it involvesWhat a predictive genetic test involves for someone with no symptoms: the appointments, the blood sample, the timing, and the choice to stop.
- The right not to knowChoosing not to find out whether you carry a family gene change is a recognised and respected choice. What it means in practice, and what stays open.
- Talking with family about genetic riskTalking with relatives about an inherited prion disease: who to tell, when, what to say to children, and what the research says about how families manage it.
- PRNP variants explainedHow PRNP variants differ: penetrance, the role of codon 129, the phenotypes they produce, and what the main variants are known to do.
- What a genetic result can and cannot tell youA genetic result says whether you carry a change in the PRNP gene. It cannot say when, or whether, someone who carries it will become ill.
Sources for this page
- MedlinePlus Genetics: Prion disease
- CDC: About Classic Creutzfeldt-Jakob Disease
- NHS: Creutzfeldt-Jakob disease (CJD), causes
- Ng M and others. Multidisciplinary recommendations for palliative and supportive care in Creutzfeldt-Jakob disease and related disorders. Age and Ageing 2026
- Kortazar-Zubizarreta M and others. The risk of transmission of genetic prion diseases is greater than 50%. European Journal of Neurology 2025