phage T7 (9) and ArdA from the conjugative transposon Tn916 (8), have been shown to comprise elongated struc- tures that bind to DNA binding site of the Mtase core of the Type I RM enzyme, thereby rendering the enzyme incapable of recognizing its DNA target both in vivo and in vitro. The structure of KlcA136 on the other hand, appears to be a rigid globular molecule of novel fold and does not mimic double-stranded DNA. ACCESSION NUMBER 2 kmg. SUPPLEMENTARY DATA Supplementary Data are available at NAR Online. ACKNOWLEDGEMENTS We thank Dr Patrick Langridge-Smith for access to the SIRCAMS FTICR-MS facility supported by the RASOR grant from the BBSRC (BB/C511599/1) and Drs Logan Mackay and David Clarke for running the FT-ICR MS. FUNDING The Biotechnology and Biological Sciences Research Council (BB/D001870/1 to D.T.F.D.); The Wellcome Trust (GR080463MA to D.T.F.D., G.W.B., Malcolm Walkinshaw and James Naismith, and G078780/Z/05/Z to D.U. and A.P.H.); The Engineering and Physical Sciences Research Council (studentship for D.S.M.); Wellcome Trust, BBSRC, Scottish Universities Life Science Alliance and The University of Edinburgh (to D.C.S. who is a part of Centre for Translational and Chemical Biology); RASOR grant from the BBSRC (BB/C511599/1 to Dr Patrick Langridge-Smith for access to the SIRCAMS FTICR-MS facility); Funding for open access charge: The Wellcome Trust. Conﬂict of interest statement. None declared. REFERENCES 1. Thomas,C.M. and Nielsen,K.M. (2005) Mechanisms of, and barriers to, horizontal gene transfer between bacteria. Nat. Rev., 3, 711–721. 2. Tock,M.R. and Dryden,D.T.F. (2005) The biology of restriction and anti-restriction. Curr. Opin. Microbiol., 8, 466–472. 3. Wilkins,B.M. 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