The intergenic Dlx5/Dlx6 forebrain enhancer was mutated (T/A>C/G) and a loxP site flanked neomycin resistance gene cassette was inserted downstream. The neo cassette was removed through subsequent cre-mediated recombination. The mutated nucleotide is ultraconserved, a polymorphism of which is associated with autism in humans. (J:312374)