The two neurofibromatoses, neurofibromatosis type 1 (NF1) and type 2 (NF2), have been greatly advanced by the cloning of their respective disease genes. Although NF1 and NF2 are clinically distinct disorders, they represent diseases caused by disruption of tumor suppressor genes. Tumor suppressor genes encode proteins involved in the proper regulation of cell differentiation and proliferation and their disruption presumably predisposes to tumor formation. This year it was demonstrated that the NF1 gene fulfills the genetic criteria for a tumor suppressor gene and that its expression is altered in a number of human malignancies unrelated to NF1. The protein product of the NF1 gene, neurofibromin, was shown to be associated with cytoplasmic microtubules and this interaction may modulate one of neurofibromin's functions. This year also witnessed the cloning of the NF2 gene and the identification of the predicted protein as a member of a family of cytoskeleton-associated proteins.