Two sets of mutations within Dscam1. The first is a 250bp deletion within the intron upstream of the exon 6 cluster of Dscam1; this deletion removes the intronic docking site that is normally used in the process that selects one exon from the exon 6 cluster during splicing, plus some downstream sequence. The second is five nucleotide substitutions within 'Ubs6.1' (upstream balancer sequence 6.1), a sequence located in the upstream intron-exon 6.1 boundary region and whose wild-type sequence is highly complementary to the conserved 'Dbs6.40' (downstream balancer sequence 6.40) intronic element located upstream of exon 6.40.