Crystal structures have now been determined for the R . PvuII restriction endonuclease as a protein-DNA complex [Cheng et al., EMBO J, 13 (1994) 3927-3935; this report] and in ape-form [Athanasiadis et al., Nature Struc. Biol. 1 (1994) 469-475; our unpublished result]. The structures indicate how the interaction with DNA might proceed [Riddihough, Nature 370 (1994) 78].