The L1L2_Bact_P cassette was inserted at position 31214943 of Chromosome 5 upstream of the critical exon 3 (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 a 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 targeted exon 3 at position 31215660. The critical exon 3 is thus flanked by loxP sites. A conditional ready (floxed) allele is created by flp recombinase expression to remove the reporter and selection cassette, leaving exon 3 floxed.