The ATG codon (Chromosome 7: 59,228,802-59,311,536; Ube3a exon 3) encoding the initiating methionine of UBE3A isoform 2 was mutated into a TGA using CRISPR/Cas9, such that only isoform 3 is generated in these mice. A diagnostic silent AscI restriction site was also introduced in exon 3. (J:261868)