Exons 2-12 (and intervening introns) of the endogenous robo3 locus have been deleted and replaced by cDNA sequence encoding robo3 lacking the Ig1 domain, tagged at the N-terminal end with 4xTag:HA. Exon 1 of the endogenous robo3 sequence remains, providing the endogenous robo3 transcription start site, ATG start codon and signal peptide, and the 16.6kb first intron is also unmodified.