An ATG codon (Chromosome 7: 59,228,802-59,311,536; Ube3a exon 4/5) encoding the initiating methionine of UBE3A isoform 3 was mutated into an alanine (GCG) using CRISPR/Cas9, such that only isoform 2 is generated in these mice. A diagnostic silent DraIII restriction site was also introduced in exon 4. (J:261868)