A loxP site was inserted upstream of exon 4, and an FRT-flanked neomycin selection cassette and a second loxP site were inserted downstream of exon 4. Flp-mediated recombination removed the selection cassette and cre-mediated recombination in the germline deleted exon 4. Deletion of exon 4 generates a frame-shift leading to disruption of the zinc-dependent metalloprotease domain and its downstream sequences.