Exon 13 of the gene was replaced with a mutated exon where the codon for Lys975 was mutated to
encode an alanine. An FRT-flanked puromycin resistance cassette was inserted adjacent to the mutation. Flp-mediated recombination removed the puromycin cassette and left a single FRT site between exon 13 and 14. This allele produces a truncated protein lacking the last 41 amino acids that is devoid of nuclease activity.