We report the isolation and characterization of a canine cDNA encoding a 216-amino acid GTP-binding protein of the Ras superfamily. The protein is almost identical to the human TC4 [Drivas et al., Mol. Cell. Biol. 10 (1990) 1793-1798] and Ran [Bischoff and Ponstingl, Proc. Natl. Acad. Sci. USA 88 (1991) 10830-10834; Nature 354 (1991) 80-82] proteins, the latter of which has been found to be involved in cell cycle control. Furthermore, the protein is highly similar to the fission yeast spi1 gene product [Matsumoto and Beach, Cell 66 (1991) 347-360]. The high degree of evolutionary conservation in this protein suggests that it plays a vital role in the eukaryotic cell.