Peptides capable of inhibiting the transforming activity of activated p21 proteins, their preparation and compositions containing them are disclosed. Particular regions of the GAP protein have been identified and characterized that are involved in the transduction of activation signals of p21 proteins. Peptides derived from GAP protein but carrying an effector region rendered non-functional have been shown to inhibit the p21 signaling route. The products according to the invention are useful in pharmaceutical compositions to inhibit the transformation activity of ras genes whose transformation proceeds by a functional p21-GAP interaction.