We have found previously undescribed CE peptidase homologs in several bacterial and in one viral species (Figure 1) . We have furthermore proven that one of the more distantly related homologs-the protein elaD in E. coli-can act as a deubiquitinating enzyme in vitro. Earlier, we had shown that related Chlamydia proteases also have deubiquitinating activity and similar observations had been made regarding the YopJ protease family. This suggests that the yet undefined bacterial CE peptidases could also display ubiquitin-or Ubl-specificity, especially considering their even closer sequence relationship to eukaryotic ULP/SENP proteases. These findings raise two important questions. First, are these proteases really specific for ubiquitin in vivo, or are we simply measuring an offtarget artifact, with the true substrates merely resembling ubiquitin? Second, why are these proteases-which are present in every eukaryote-so widely distributed in bacteria and viruses as well, and what can we learn about their genetic origin?