Recent evidence associates the codon 12 valine-for-glycine (G12V) mutant Ki-Ras protein with higher stage and increased lethality of colorectal carcinomas, while the codon 12 aspartate-for-glycine (G12D) Ras mutation shows no such association. Several observations may be relevant to this phenomenon. First, GTPase activity of G12V Ras is one-quarter that of G12D Ras and one-tenth that of wild-type (WT) Ras. Second, binding of the GTP analogue GppNp to G12D Ras is 8-fold weaker than its binding to G12V or WT Ras and crystal structures indicate that electrostatic repulsion between the carboxylate group of the G12D Asp-12 side-chain and the gamma phosphate of the bound nucleotide may make GTP binding to G12D Ras weaker even than that of GppNp. It is proposed that this lowering of affinity for GTP allows G12D Ras an escape from the oncogenic GTP-bound state, whereas GTP tightly bound to G12V mutant Ras generates a more persistent, potentially oncogenic, signal. Structural comparisons also suggest that differences between the Switch I (effector) region of G12D and G12V Ras could modify interactions with downstream signalling molecules such as Raf-1, neurofibromin, and phosphatidylinositol 3-hydroxy-kinase. Other differences between the G12D and G12V mutant Ras proteins include a lower affinity of the GTPase activating protein GAP for G12V than for G12D or WT Ras; but, as both G12D and G12V Ras are refractory to GTPase activation by GAP binding, this may be less significant. These studies complement experimental data showing that such Ras mutations differ in their effects in vitro and in vivo and, with recent data indicating heterogeneity of ras mutation in colorectal carcinomas and other tumours, make it plausible that codon 12 Ras mutations differ in carcinogenic potential and prognostic significance.