Open the Ensembl genome browser (http://www.ensembl.org/Danio_rerio/Info/Index) and search for the target gene using the zebrafish genome. On the target gene page, choose ‘Transcript ID’ for each transcript variant. In the ‘Transcript-based displays panel’ on this page, select ‘Exons’ under ‘Sequence’ to obtain sequence information and identify the translated exon containing the ATG start codon. Repeat this step for all variants and identify the ATG exon included in all potential transcript variants. Any introns located downstream of this ATG exon can be used for Zwitch insertion. The exon usage may need to be experimentally confirmed using target cells and/or tissue samples.