Ras p21 can exist in either a physiologically quiescent GDP-binding state or a GTP-binding signal-emitting state (1,2). Interaction of Ras p21 with GTPase activating protein (GAP) can increase the rate of hydrolysis of Ras p21-bound GTP by as much as 1000-fold (3). In mitogenically activated and tyrosine kinase-transformed cells, Ras GAP forms a complex with a protein designated p190 (4). At its amino terminus, p190 contains sequence motifs characteristic of all known GTPases, whereas the carboxy terminus contains sequences similar to those found in the Bcr gene product, n-chimerin and Rho GAP, all of which exhibit intrinsic GAP activity (4,5). Oligophrenein-1 is an additional protein with GTPase activating activity. Oligophrenein-1 is a RhoGAP protein that stimulates GTP hydrolysis of Rho subfamily members and is involved in cell migration, morphogenesis and axon outgrowth (6).