Clonal overexpression of robo3^{Scer\UAS.T:Ivir\HA1,T:SS-wg} under the control of Scer\GAL4^sens.PB using the MARCM technique results in abnormalities in R8 axon projections.

Expression of robo3^{Scer\UAS.T:Ivir\HA1,T:wg} under the control of Scer\GAL4^109(2)80 results in axon mistargeting and ectopic branching of class IV dendritic arborization (da) neurons, primarily along the VL fascicle (this is the normal position of collaterals of class II da neuron axons).

Clones in the nervous system expressing robo3^{Scer\UAS.T:Ivir\HA1,T:wg} under the control of Scer\GAL4^109(2)80 result in defects in axon morphology in class II, II and IV da neurons. The class II neuron ldaA truncates its arbor before reaching its usual position. Class III and IV neurons show ectopic collateral branching. The class IV neuron v'ada reaches a medial position comparable to wild-type class IV neurons, but projects laterally in the next anterior segment.

The antennal commissure forms normally robo3^{Scer\UAS.T:Ivir\HA1,T:wg}; Scer\GAL4^SG18.1 flies, but most sensory neuron terminals are shifted dorsomedially within the antennal lobes. In the antennal lobes of robo3^{Scer\UAS.T:Ivir\HA1,T:wg}; Scer\GAL4^lz-gal4 flies, sensory neurons terminate at ectopic positions and often show altered morphologies. Arborization of these neurons is most frequently shifted to a mediodorsal axis and large irregular-shaped glomeruli are frequently formed. These ectopic glomeruli show normal organisation of cellular components.

Expression of one copy of robo3^{Scer\UAS.T:Ivir\HA1,T:wg} under the control of Scer\GAL4^ap-md544 shifts ap-expressing neurons (which normally project along the medial edge of the longitudinal tract) laterally. The ap-expressing fascicle often splits into 2 or 3 parallel pathways. Expression of two copies of robo3^{Scer\UAS.T:Ivir\HA1,T:wg} under the control of Scer\GAL4^ap-md544 shifts the neurons even further laterally.

When two copies of P{UAS-robo3.HA1} are driven by Scer\GAL4^elav.PLu commissure formation is prevented. Commissure formation is not prevented when only a single copy of P{UAS-robo3.HA1} is present. In robo3 mutants no pCC or aCC growth cones stray from their normal pathways.