The L1L2_Bact_P cassette was inserted at position 120419414 of Chromosome 4 upstream of the critical exon 4 (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 4 at position 120420614. The critical exon 4 is thus flanked by loxP sites. The lacZ and neomycin resistance gene were removed via flp-mediated recombination. Exon 4 was deleted via cre-mediated recombination generating a knock-out allele.