The L1L2_Bact_P cassette was inserted at position 23240733 of Chromosome 19 upstream of 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 exon 4 at position 23241614. Exon 4 is thus flanked by loxP sites. A null/knockout allele was created by flp- and cre-mediated recombination to remove the lacZ sequence, neo cassette and the floxed exon 4.