The L1L2_gt1 cassette was inserted at position 149557403 of Chromosome 4 upstream of the critical exon 2 (Build GRCm39). The cassette is composed of an FRT flanked lacZ/neomycin sequence followed by a loxP site. An additional loxP site is inserted downstream of the targeted exon(s) at position 149558354. The critical exon 2 is thus flanked by loxP sites. A "conditional ready" (floxed) allele was created by flp recombinase expression in mice carrying this allele to remove the lacZ sequence and neo selection cassette, leaving loxP sites flanking the critical exon(s). Subsequent cre-mediated recombination deleted exon 2 and resulted in a knockout mouse.