Mutations have have been introduced into the D. virilis Dbs6.44 (downstream balancer sequence 6.44) and Dbs6.48 (downstream balancer sequence 6.48) intronic elements located respectively upstream of exon 6.44 and 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).