Abstract: Ras proteins are small GTPases ivolved in signaling pathways controlling cell growth and differentiation. They act as molecular switches by cycling between an active GTP- and an inactive GDP-bound state. Following the activation of specific cell-surface receptors, Ras proteins switch from inactive to active state through the catalytic action of specific Guanine nucleotide Exchange Factors (GEFs), that promote the dissociation of GDP from Ras, allowing GTP entrance into the Ras nucleotide poket. The Saccharomyces cerevisiae Ras-GEF Cdc25 (Cdc25Sc) was the first Ras-exchanger to be identified. In higher eukaryotes there are two different classes of Ras-specific Cdc25Sc homologs, Sos proteins and Cdc25Mm, also referred to as Ras GRF. Ras-specific GEFs are made of several functional and structural domains, Ras GEF activity is contained within a domain showing very high similarity to the Cdc25Sc catalytic domain and called, for this reason, Cdc25 homology domain. Structural studies on Ras crystallized in complex with nucleotide (GDP or GTP-analogs) and human exchange factor Sos respectively have allowed both to identify conformational differences between active and inactive state of Ras, and to make hypothesis on molecular determinants of interaction and catalytic activity of human Sos. Mutational and structural studies on Ras GEFs catalytic domain have pointed to a major role for the helical-hairpin formed by αH and αI helixes (catalytical hairpin) in the catalytic mechanism of Ras-specific GEFs. In the present work we investigate the Ras GEF αG-helix role in Ras-GDP to GTP exchange.