The mammalian proto-oncogene Ras encodes 21 kDa proteins (p21Ras) which exists in either a physiologically quiescent GDP-binding state or a GTPbinding signal-emitting state. Interaction of p21Ras with GTPase activating protein (GAP) can increase hydrolysis of p21Ras-bound GTP by as much as 1000-fold. The product of the neurofibromatosis type 1 gene (NF1) has also been shown to exhibit p21Ras GAP activity.