The L1L2_Bact_P cassette was inserted at position 170965856 of Chromosome 1 upstream of exon 4 (Build GRCm39). The cassette is composed of an FRT site followed by lacZ sequence and a loxP site. This first loxP site is followed by neomycin resistance gene 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 the exon 4 at position 170966998. The critical exon 4 is thus flanked by loxP sites. A "conditional ready" (floxed) allele was created by flp recombinase expression in mice, leaving exon 4 flanked by loxP sites.