Neurofibromatosis (NF) is one of the most common genetic conditions. There are several different types: about one in 2500 people have neurofibromatosis type 1 (NF1). This causes many benign nerve tumours, which can occur anywhere in the body. They first appear during childhood as coffee-coloured spots on the skin. Later in life, more nerve tumours appear as lumps under the skin. NF1 sometimes causes mild learning difficulties. Symptoms of NF1 vary a lot, even between affected people in the same family.