The targeting vector is designed to insert downstream of the first non-coding intron with (from 3' to 5') an FRT site, engrailed 2 intron and exon with splice acceptor site, H2B-Venus-polyadenylation sequence, rox-flanked beta actin promoter driven NEO cassette, and a loxP site, followed by exon 2 and a second loxP site. Two alternative translation initiation codons are present in exon 2.