The mammalian c-H-, c-K- and N-Ras proto-oncogenes encode ubiquitously expressed proteins (1,2). p21Ras can exist in either a physiologically quiescent GDP-binding state or a GTP-binding signal-emitting state (3). Oncogenic p21Ras proteins are trapped in the excited signal-emitting state because the mechanism normally employed to delimit their excitation period, hydrolysis of their bound GTP to GDP, is impaired as a result of specific mutations (3). Interaction of p21Ras with GTPase activating protein (GAP) can increase hydrolysis of p21Ras-bound GTP by as much as 1000-fold (4,5). The product of the neurofibromatosis type 1 gene (NF1) has also been shown to exhibit p21Ras GAP activity (6,7), and proteins that stimulate the GTPase activity of three other low molecular weight GTPases, including Rho, Rab 3A and Rap 1, have also been described (8,9).