The L1L2_Bact_P cassette was inserted at position 38997948 of Chromosome 2 upstream of critical exon 7 (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 7 at position 38998905. Exon 7 is thus flanked by loxP sites. The exon and neo cassette were deleted through subsequent cre-mediated recombination.