The L1L2_Del_BactPneo_FFL cassette was inserted at position 27396587 of Chromosome 19 upstream of exon 1 (Build 37). The cassette is composed of an FRT site followed by a neomycin resistance gene under the control of the human beta-actin promoter, SV40 polyA, a second FRT site and a loxP site. A second loxP site is inserted downstream of exon 1 at position 27398917. Exon 1 is thus flanked by loxP sites. Exon 1 was deleted through subsequent cre-mediated recombination.