We describe here the cloning and sequencing of human and mouse cDNAs encoding a putative GTP binding protein. Sequence comparison shows that these cDNAs (named eRFS) are likely to represent the orthologues of the yeast Saccharomyces cerevisiae HBS1 gene and that the C-terminal domains of the encoded proteins share structural features with eukaryotic elongation factor eEF-1A and release factor 3 (eRF3) families. The phylogenetic analysis suggests that eRFS proteins and Hbs1p form a cluster of orthologous sequences branching with the eRF3 family. Nevertheless, in yeast, the human eRFS protein and Hbs1p do not complement eRF3/Sup35p thermosensitive mutation and do not interact with eRF1. (C) 1998 Federation of European Biochemical Societies.