Deletion of 'Ubs6.1' (upstream balancer sequence 6.1), a sequence located in the upstream intron-exon 6.1 boundary region of Dscam1 and whose wild-type sequence is highly complementary to the conserved 'Dbs6.40' (downstream balancer sequence 6.40) intronic element located upstream of Dscam1 exon 6.40.