The L1L2_gt0 cassette was inserted at position 113357242 of Chromosome 6 upstream of the critical exon (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 exon 2 at position 113358083. 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 exon 2 flanked by loxP sites.