Neurofibromatosis type 1 (NF1) is a common monogenic tumor-predisposition disorder with an autosomal dominant pattern of inheritance that arises secondary to haploinsufficiency of the tumor suppressor gene NF1. Haploinsufficiency is often defined as a gene-dosage effect in which loss of one allele of a gene results in disease. The gene discussed is NF1; the disease is neurofibromatosis.