The L1L2_gt1 cassette was inserted at position 167282591 of Chromosome 2 upstream of the critical exon 4 (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 4 at position 167283331. Exon 4 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 exon 4.