The gene for von Recklinghausen neurofibromatosis type 1 (NF1) was recently identified by positional cloning and found to encode a protein with sequence similarity to a family of eucaryotic GTPase-activating proteins (GAPs). Expression of the NF1-GAP-related domain (NF1GRD) has been shown to complement yeast strains deficient in the yeast GAP homologs, IRA1 and IRA2, to interact with human RAS proteins and to accelerate the conversion of ras-GTP to ras-GDP. Further analysis of this region has revealed a number of residues that are highly conserved between members of the GAP family. Mutational analysis of a representative number of these residues produced one of three effects: (1) no change in NF1GRD function, (2) complete disruption of NF1GRD function and (3) intermediate retention of NF1GRD function. One of these mutations at residue 1423 was shown to have reduced ability to negatively regulate ras in yeast, which is interesting in light of a recent report demonstrating a similar naturally occurring mutation in human malignancies.