The mammalian Grf1 and Grf2 proteins are Ras guanine nucleotide exchange factors (GEFs) sharing a high degree of structural homology, as well as an elevated expression level in central nervous system tissues. Such similarities raise questions concerning the specificity and/or redundancy at the functional level between the two Grf proteins. grf1-null mutant mice have been recently described which showed phenotypic growth reduction and long-term memory loss. To gain insight into the in vivo function of Grf2, we disrupted its catalytic CDC25-H domain by means of gene targeting. Breeding among grf2+/− animals gave rise to viable grf2−/− adult animals with a normal Mendelian pattern, suggesting that Grf2 is not essential for embryonic and adult mouse development. In contrast to Grf1-null mice, analysis of grf2−/− litters showed similar size and weight as their heterozygous or wild-type grf2 counterparts. Furthermore, adult grf2−/− animals reached sexual maturity at the same age as their wild-type littermates and showed similar fertility levels. No specific pathology was observed in adult Grf2-null animals, and histopathological studies showed no observable differences between null mutant and wild-type Grf2 mice. These results indicate that grf2 is dispensable for mouse growth, development, and fertility. Furthermore, analysis of double grf1/grf2 null animals did not show any observable phenotypic difference with single grf1−/− animals, further indicating a lack of functional overlapping between the two otherwise highly homologous Grf1 and Grf2 proteins.This work was supported by FEDER grants 1FD97-1735 and 1FD97-1678 and CICYT grant SAF00-0069 from MCYT (Spain). A.F.-M. and L.M.E. contributed equally to this work.Peer reviewed