The mutated allele contains a loxP site immediately upstream of an exon 16 sequence modified by the insertion of a FLAG motif and a stop codon just downstream of the beta secretase cleavage site. The mutated copy of exon 16 was separated from a second, intact, copy of exon 16 by a sequence cassette containing two copies of neomycin flanked by FRT sites and followed by a second loxP site. Flp recombinase was then used to excise the two neomycin sequences leaving the mutated exon 16 surrounded by loxP sites. Transmembrane and intracellular sequences are deleted from the translation product.