Exons 2-12 (and intervening introns) of the endogenous robo3 locus have been deleted and replaced by cDNA sequence encoding robo3 in which the Ig1 domain has been replaced with the Ig1 domain from the orthologous Mmus\Robo3 gene, with the coding sequence 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.