The RMCE cassette present in TI{TI}Dscam1^KO.RMCE has been replaced by sequence encoding Dscam1 exons 3-5 in which all variants of exon 4 have been mutated by a single base pair insertion, and 2.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 negative control, as splicing of any of the variants of exon 4 results in a frameshift that generates a stop codon after exon 5.