Reproducing the naturally occurring O-2-tolerant hydrogenases is a potential strategy to make the oxygen sensitive enzymes, produced by organisms of biotechnological interest, more resistant. The search for resistance "hotspots" that could be transposed into sensitive hydrogenases is underway. Here, we replaced two residues (Y77 and V78) of the oxygen sensitive [NiFe] hydrogenase from Desulfovibrio fructosovorans with Gly and with Cys, respectively, to copy the active site pocket of the resistant membrane-bound [NiFe] enzyme from Ralstonia eutropha and we examined how this affected oxygen sensitivity. The results are discussed in the light of a short review of the recent results dealing with the reactivity of hydrogenases towards oxygen. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.