The KOMP targeting vector was inserted at position 122156726 of Chromosome 9 upstream of exon 4 (Build GRCm38). The cassette is composed of an FRT site followed by lacZ sequence, a loxP site, neomycin resistance gene under the control of the human beta-actin promoter, SV40 polyA, a second FRT site and a second loxP site. The third loxP site is inserted downstream of exon 4 at position 122157552. Flp-mediated recombination removed the FRT-flanked lacZ/neo cassette leaving exon 4 floxed.