A 1.8 kb cDNA clone, Krev-1, with revertant-inducing activity on Kirsten sarcoma virus-transformed NIHI 3T3 cells, has been isolated from a human fibroblast cDNA expression library. In Krev-1 transfectants, there is a correlation between the levels of specific mRNA and the degrees of suppression of the transformed phenotype. The cDNA encodes a protein of 21,000 daltons that unexpectedly shares around 50% amino acid identities with ras proteins. The Krev-1 homologs are found in mouse, rat, and chicken DNA, and their transcripts are ubiquitously expressed in many rat organs. Thus, the Krev-1 gene seems to play an important role (s) in a wide variety of tissues, and may be involved in the negative growth regulation of certain cell types. dimensional structure of the p21 Ha-ras-GDP complex, as determined by X-ray crystallography (de Vos et al., 1988). On the other hand, only a limited amount of information concerning the physiological roles of ras proteins in normal cells, as well as in the process of carcinogenesis, has been reported. As an approach toward understanding the mechanism of cell transformation induced by activated ras genes, we have been characterizing morphologically nontransformed(“flat”) revertants with greatly reduced malignancy (Noda et al., 1983; Bassin and Noda, 1987) from a transformed subline of NIH13T3 (the “DT” cell line) containing Kirsten murine sarcoma virus, a transforming virus carrying the v-Ki-ras gene. In a previous report, we described the isolation of seven flat revertants from DT cells following transfection of a human fibroblast cDNA expression library and various enrichment procedures (Noda et al., 1988). It is interesting that each revertant seemed to contain dominant suppressor activity against specific sets of oncogenes (unpublished data). This and other observations are consistent with the idea that we can detect, by this approach, multiple cDNA species expressed in normal human fibroblasts that have the potential for suppressing the transformed phenotype associated with activated ras genes and some other oncogenes. Here, we describe a human cDNA, Krev-1, recovered from one of the seven revertants, R16, which exhibits resistance specific to activated ras genes. Krev-1 suppresses some, if not all, of the transformed phenotype of DT cells when expressed at high levels and, unexpectedly, encodes a novel protein with structural similarities to ras proteins.