Four families of transporters have previously been described that contain multidrug efflux systems: the major facilitator superfamily (MFS), the small multidrug resistance (SMR) family, the resistance/nodulation/cell division (RND) family and the ATP‐binding cassette (ABC) superfamily. The MFS, SMR and RND families consist of secondary transporters, typically energized by the proton‐motive force (pmf), and the ABC superfamily comprises ATP‐dependent transporters (Paulsen et al., 1996, Microbiol Rev60: 575–608). Recently, two new multidrug efflux proteins have been identified, NorM from Vibrio parahaemolyticus, and a homologue in E. coli, YdhE (Morita et al., 1998, Antimicrob Agents Chemother42: 1778–1782). These two proteins mediate resistance to a range of cationic dyes, aminoglycosides and fluoroquinolones, probably via a pmf‐dependent efflux mechanism. NorM and YdhE have 12 predicted transmembrane segments (TMS) and on this basis have been suggested to be members of the MFS (Morita et al., 1998, ibid.).