The L1L2_gt0 cassette was inserted at position 56796283 of Chromosome 4 upstream of the critical exon(s) (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 targeted exon(s) at position 56796283. The critical exon is thus flanked by loxP sites. Flp mediated recombination removed the lacZ/neomycin cassette leaving exon 4 floxed. 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.