Background: Neurofibromatosis type 1 (NF1) is a genetic disease that can cause many symptoms. About half of people with NF1 will develop benign (noncancerous) tumors along nerves in the skin, brain, and other parts of the body. Sometimes, though, these tumors can become cancerous. Researchers do not yet know how to predict which tumors will become cancerous.

Objective: To test a new method for predicting which benign NF1 tumors will become cancerous.

Eligibility: People aged 3 years and older with a clinical or genetic diagnosis of NF1.

Design: * Participants will be screened with a review of their medical history. All participants will have a baseline visit. They will have bood tests and imaging scans. They will have a physical exam. They will answer questions about their family history. Participants aged 8 years and older will take tests of their thinking skills and their emotional health. * Some participants may be asked to undergo more tests. These may include another type of imaging scan and a biopsy: A small sample of tissue may be removed from the tumor. * Participants will be divided into two groups: those believed to be at low risk and those believed to be at high risk of developing cancer. * Participants in the high-risk group will be asked to return for their next visit in 1 month to 3 years. * Participants in the low-risk group will be asked to return for their next visit in 6 months to 5 years. * Participants may also have follow-up visits by phone throughout the study. They will be in the study for 10 years.