To determine the role of the catalytic activity of the OTU domain in IFN antagonism, we changed the cysteine at position 40 and the histidine at position 151, two components of the catalytic triad, to alanine (C40A and H151A respectively). A third mutant was created where glutamine 16 was replaced by arginine (Q16R); Q16 has been described as important for the binding of the CCHFV OTU to ubiquitin but with little importance for the binding of ISG15 [44, 45] , so this mutation was designed to allow us to study solely the effects of deISG15ylation on the innate immune system. The mutations were introduced individually into the GV L1-169 construct, and transfected into 293FT cells to examine their effect on ubiquitination and ISGylation ( fig. 8a, b) . Mutation of C40 or H151 resulted in a complete loss of deubiquitinating ( fig. 8a) and deISG15ylating (fig. 8b) activity. The Q16R mutant performed like the wildtype regarding its ability to remove ISG15 from cellular substrates ( fig. 8b ) while its deubiquitinating activity was only slightly reduced when compared to the wildtype OTU ( fig. 8a) . In previous studies on CCHFV a similar mutant was described as being unable to hydrolyze a fluorogenic model DUB substrate (Ub-AMC) [44] . The difference might be explained by the fact that we are using the OTU domain from a different virus and also a different assay for DUB activity.