CF develops when both copies of the CF gene a person has inherited (one copy from each of the parents) is faulty. Faulty genes are called ‘mutations’. If genes are thought of as being codes for instructions to parts of the body to work in a certain way, then gene mutations can be though of as being like typing errors in that code. People with only one faulty copy of a CF gene are called ‘CF carriers.’ About 1 out of every 25 people in the UK is a carrier. Carriers are generally healthy and do not have Cystic Fibrosis. If two CF gene carriers have a baby together then there is a 1 in 4 chance that the child will inherit the faulty gene from both their mother and father and so have CF. But because there are many ‘typing errors’ causing different mutations in the CF gene, then the clinical effects of CF can be quite varied from person to person. Every person with CF is unique.