A targeting construct was designed to insert a C to T point mutation in exon 5 of the gene, resulting in a non-sense mutation, R151X, commonly found in humans with neuronal ceroid lipofuscinoses (NCLs). A loxP- and frt-flanked neomycin (neo) resistance cassette was inserted downstream of exon 5. Cre-mediated recombination removed the floxed neomycin cassette.