The L1L2_gt0 cassette was inserted at position 74248471 of Chromosome 4 upstream of the critical exon 9 (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 exon 9 at position 74249363. Exon 9 is flanked by loxP sites. A null allele was created by deleting exon 9 through cre-mediated recombination.