The RMCE cassette present in TI{TI}Dscam1^KO.RMCE has been replaced by sequence encoding Dscam1 exons 3-5 in which 1. all variants of exon 4, except exon 4.5 have been mutated by a single base pair insertion, and 2. sequence encoding the Tag:M(mCd8a) transmembrane domain, a T2A self-cleaving peptide and the GAL4 driver followed by a stop codon and the SV40 polyA site has been fused to the C-terminal end of exon 5. This transgene acts as a reporter of exon 4.5 splicing, as splicing of any alternative exon other than 4.5 results in a frameshift that generates a stop codon after exon 5. Upon splicing of alternative exon 4.5 and generation of an in-frame coding sequence, the presence of the T2A peptide allows the GAL4 driver to be translated separately from the non-functional Tag:M(mCd8a)-tagged Dscam1 exon 1-5 peptide fragment.