The cause is a genetic mutation in certain oncogenes. These can be inherited from a person's parents, or in about half of cases spontaneously occur during early development. Different mutations result in the three types of NF. Neurofibromatoses arise from the supporting cells of the nervous system rather than the neurons themselves. In NF1, the tumors are neurofibromas (tumors of the peripheral nerves), while in NF2 and schwannomatosis tumors of Schwann cells are more common. Diagnosis is typically based on symptoms, examination, medical imaging, and biopsy. Genetic testing may rarely be done to support the diagnosis.