The L1L2_Bact_P cassette was inserted at position 67398012 of Chromosome 3 upstream of the critical exon 3 (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 3 at position 67398906. The critical exon is thus flanked by loxP sites. Flp-mediated recombination removed the lacZ and neomycin resistance gene and cre-mediated recombination deleted exon 3. Further information on targeting strategies used for this and other IKMC alleles can be found at http://www.informatics.jax.org/mgihome/nomen/IKMC_schematics.shtml.