The construct is generated from a L1L2_Bact_P cassette inserted at position 131797953 of Chromosome 1 upstream of exon 4. The cassette is composed of an FRT site followed by lacZ sequence and a loxP site. This first loxP site is followed by neomycin under the control of the human beta-actin promoter, SV40 polyA, a second FRT site and a second loxP site. A third loxP site is inserted downstream of exon 4. Cre-mediated recombination removed the neo cassette and exon 4..