The modified gene contains a loxP site upstream of exon 5, a single frt site (left by FLP recombinase excision of a neomycin resistance cassette) followed by a second loxP site downstream of the 3' UTR, and a second copy of exon 5 followed in-frame by the EGFP coding sequence. Cre recombinase-mediated excision of the endogenous exon 5 through the 3' untranslated region will result in splicing of exon 4 to the downstream copy of exon 5, leading to production of a SAMD8/EGFP fusion protein lacking the catalytic domain encoded by exon 6.