The exons (and intervening small introns) that encode the mature robo2 protein have been replaced with a single exon encoding full length robo1 tagged with Tag:HA. The robo2KO.robo1 allele represents the loss of the robo2 open reading frame at the endogenous robo2 locus, while the robo1robo2.HR.Tag:HA allele represents the mature robo1 protein tagged with Tag:HA that is expressed under the control of the robo2 regulatory sequences in place of robo2.
robo1robo2.HR.Tag:HA, robo2KO.robo1 has viable phenotype
The defects in the lateral Fas2-expressing longitudinal axons that are seen in lea mutant embryos are not rescued in leaKO.robo robolea.HR.T:Ivir\HA1 double homozygous embryos (33.0% of hemisegments have fused or broken lateral Fas2-positive fascicles). Fas2-positive longitudinal axons cross the midline in only 2.0% of segments in the leaKO.robo robolea.HR.T:Ivir\HA1 double homozygous embryos (compared to in 23.7% of segments in lea null mutant embryos).
The characteristic robounspecified phenotype (crossing of the midline by Fas2-positive longitudinal axons) is not rescued if the embryos are also heterozygous for robolea.HR.T:Ivir\HA1.