The L1L2_Bact_P cassette was inserted at position 16097421 of Chromosome 1 upstream of exon 2 (Build 37). The cassette is composed of an FRT site followed by lacZ sequence and a loxP site. This first loxP site is followed by 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 2 at position 16098357. The critical exon(s) is/are thus flanked by loxP sites. Flp and Cre mediated recombination removed the lacZ and neo cassettes and exon 2. Quantitative RT-PCR indicates transcript levels are at about 50% that of wild-type in homozygous embryos.