Mutations have have been introduced into the D. virilis Dbs6.48 (downstream balancer sequence 6.48) intronic element located upstream of exon 6.48 of the Dscam1^CK locus. (Dscam1^CK is a chimeric locus in which the distal region of the D. melanogaster Dscam1 exon 6 cluster, extending from a point in the intron downstream of exon 6.35 through to a point in the intron downstream of exon 6.48, has been replaced with the corresponding sequence from D. virilis which extends from a point in the intron downstream of exon 6.41 to a point in the intron downstream of exon 6.52).