The 83 amino acid exon 15 and 16 sequence in robo1 has been replaced by an altered nucleotide sequence that produces the same amino acid sequences as in wild type robo1. A single loxP site is present in the intron after exon 14.
roboCS mutant flies are viable and fertile. The CNS of roboCS mutant embryos does not show any phenotypic abnormality. Commissure formation and distribution occurs normally in all embryos.