Homing endonucleases are encoded by mobile DNA elements that are found inserted within host genes in all domains of life. The crystal structure of the homing nuclease PI-Sce [1] revealed two domains: an endonucleolytic centre resembling the C-terminal domain of Drosophila melanogaster Hedgehog protein, and a second domain containing the protein-splicing active site. This Domain corresponds to the latter protein-splicing domain.

1: Crystal structure of the intein homing endonuclease PI-SceI bound to its recognition sequence. Moure CM, Gimble FS, Quiocho FA; Nat Struct Biol 2002;9:764-770. PMID:12219083