Group II introns contain a large ribozyme, which catalyzes self-splicing, and the coding sequence of a reverse transcriptase, the function of which is to cooperate with the ribozyme to achieve genomic mobility. Despite its lack of substrates for both steps of the splicing process, the crystal structure of a group II ribozyme reveals the location of two metal ions most likely to be involved in catalysis; the RNA structure that binds to these ions results from the bending of a local motif by the folding of the rest of the ribozyme. The stage is now set to determine where the intron-encoded protein binds to its partner and whether the spliceosome uses a counterpart of the group II catalytic center to excise nuclear pre-messenger introns.