Neurofibromatosis is not a single medical illness, but rather three distinct disorders characterized by the growth of tumors in the brain, spinal cord, and nerves that transmit signals between the brain and spinal cord and the rest of the body. These tumors are called neurofibers. There are two main types of neurofibers: neurofibrosarcomas and neurofibroma. Neurofibrosarcoma is a malignant tumor that develops from connective tissue (muscle, fat, or bone) surrounding a nerve or from soft tissue such as muscle or skin. It is most common among people with NF1. The second type, neurofibroma, is a benign tumor that develops from nerve cells (schwann cells), their supporting tissue (collagen and elastin fibers), and blood vessels. Neurofibromas can grow large enough to press on major blood vessels and cause problems for those tissues they protect. However, most neurofibromas are small and not cause any symptoms. People with neurofibromatosis have an increased risk of developing other diseases, especially cancer. Cancer is the most serious long-term complication for individuals with this condition. The most common cancers associated with neurofibromatosis are: breast cancer for women and schwannoma of the heart for men.