The exons (and intervening small introns) that encode the mature robo3 protein have been deleted from the endogenous robo3 locus and replaced by cDNA encoding the Tcas\robo2-3 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. The robo3KO.Tcrobo2-3 allele represents the loss of the robo3 open reading frame at the endogenous robo3 locus, while the Tcas\robo2-3HA allele represents the mature Tcas\robo2-3 protein tagged with Tag:HA that is expressed under the control of the robo3 regulatory sequences in place of robo3.