Neurofibromatosis 1 serves as a paradigm for understanding the principles of human genetics. The concepts of gene mutation, penetrance of the condition, variable clinical expressivity, mosaicism, age-dependent expression of clinical manifestations, and pleiotropy are evident in this autosomal dominant condition. The lack of genotype-phenotype correlation, except the whole-gene deletion phenotype, leads to speculation on modifiers of the haploinsufficient state of the NF1 gene product neurofibromin. The variant form of neurofibromatosis, neurofibromatosis Noonan's syndrome, suggests potential interaction of independent biochemical pathways. Identification of the NF1 gene led to the discovery of its role in ras signal transduction. Neurofibromin is a negative regulator of intracellular ras signaling. This observation now provides the framework for the development of rational medical therapies. In addition, knowledge of the molecular basis of the variable expression of clinical manifestations could provide better anticipatory guidance and more effective management of the medical complications that are associated with this condition.