dock3 cknK.Δ324-331 double homozygotes exhibit delayed 'immature' ISNb axons in 65% of hemisegments. Motor axons in the affected nerves are loosely organised with multiple projections and resemble wild-type axons at earlier stages. Furthermore, the ISNd branch is frequently absent or reduced in size. Examination of the ISNb/d choice point reveals defects in ISNb/d branch segregation. The lateral two longitudinal Fas2-positive fascicles are poorly fasciculated and discontinuous in these double mutants.