Neurofibromatosis type 1 (NF1) is caused by a genetic mutation in a gene called NF1.
Genes are the instructions used to produce all human characteristics. A genetic mutation is a permanent change in the DNA sequence that makes up a gene.
Normally, the NF1 gene is responsible for producing a protein that helps regulate the growth of nerve tissue. The protein "switches off" the growth of tissue once it's reached a certain size.
In someone with NF1, this gene is faulty. This leads to the cells producing an incomplete protein that's much less effective at switching off the growth of nerve tissue, leading to uncontrolled growth (tumours) developing in the nervous system.
The faulty gene that causes NF1 is found on chromosome number 17.
In around half of all cases of NF1, the mutated gene is passed down from a parent to their child.
The NF1 mutation is known as an autosomal dominant mutation. This means only one parent has to have the faulty gene for a child to be at risk of developing the condition.
If either the mother or father has the faulty gene, there's a one in two chance each child they have will develop NF1. Males and females have the same chance of developing the condition.
The severity of the condition isn't always inherited. For example, a child may have mild NF1 even if they inherited the condition from one of their parents who's more severely affected.
In around half of NF1 cases, the mutation appears to happen spontaneously in either the sperm or egg just before conception (sporadic mutation). It's unclear what causes it or whether anything increases the risk of it happening.
If you have a child who develops NF1 as a result of a spontaneous mutation, it's highly unlikely any further children you have will also develop the condition.
A person who develops NF1 sporadically can pass the condition to their children. The chances of this happening are the same as for someone who inherited the condition themselves (see above).