Elucidation of the partial genomic structure and DNA sequence of the gene that is altered in neurofibromatosis type 1, and the discovery of clues to its function, have opened new opportunities not only for understanding this particular disease process but also for clarifying signal pathways involved in cellular growth and differentiation. (This review is an updated and modified version of a review first published in Current Opinion in Genetics and Development 1991, 1:15-19.)