A short 'isoTarget knockout' (iso-KO) cassette has been inserted into the endogenous Dscam1 locus, in the juxtamembrane domain in exon 17.1. Exons 17.1 and 17.2 of Dscam1 are mutually exclusive, encoding two different transmembrane and juxtamembrane domains (TM1 and TM2); exon 17.1 encodes the TM1 alternative form. The iso-KO cassette consists of 1. a short RSRT stop cassette containing multiple termination codons to ensure disruption of translation and 2. a Tag:HA epitope. In the absence of RSR recombinase, this results in loss of function of only TM1-containing isoforms of Dscam1.