Exons 2-14 of the endogenous robo2 locus have been replaced by robo2 cDNA sequence that lacks the Ig1 domain and in which the coding sequence is tagged at the N-terminal end with 4xTag:HA. The endogenous robo2 promoter, transcriptional start site, first exon (including the start codon and signal sequence) and first intron remain unmodified.