The ras proto-oncogene1, found in all eukaryotes so far examined2-9, encodes a protein with guanine nucleotide-binding and GTPase activity10-13. Gene disruption experiments in yeast indicate that ras is essential for cell growth14. Anti-sense mutagenesis approaches suggest that this is also true for Dictyostelium15. Most mutations causing an amino-acid substitution for Gly 12 result in decreased GTPase activity and produce a transforming phenotype16-19. In yeast, a Gly 19 → Val 19 missense mutation (Gly 19 is similar to Gly 12 in mammalian and Dictyostelium ras proteins) causes a series of dominant phenotypes, including elevated adenylate cyclase activity20. In mammalian cells there is no evidence that ras activates adenylate cyclase activity21. D. discoideum contains a single ras gene (Dd-ras) that encodes a protein very similar to the mammalian ras protein4 and identical to c-ras at the potentially transforming positions. Dd-ras is expressed in vegetative cells and later in development in prestalk cells whereas ras protein is found in vegetative and developing cells. In the migrating pseudoplasmodium, ras protein is found in prestalk but not prespore cells, suggesting it is involved in the function and/or differentiation of the anteriorly localized prestalk cells. In this report we examine the effects of expression of a Dd-ras gene carrying a Gly 12 → Thr 12 missense mutation. © 1986 Nature Publishing Group.