A loxP site was inserted upstream of exon 38 and an FRT flanked neomycin resistance cassette with a 5' loxP site was inserted downstream of exon 41. Flp-mediated recombination removed the selection cassette and left exons 38 through 41 floxed. Cre-mediated recombination removed exons 38 through 41, encoding two of the three receptor interacting domains (N2 and N3).