retn^89.GAL4/retn^z2-428 flies eclose with only 33% of expected rates. retn^89.GAL4/retn^z2-428 animals also exhibit mushroom body (MB) specific abnormalities. MB neurons diverge within the nerve tracks and axons from the posterior part of the β-lobe neurons cross the midline and join with the opposite β-lobe neurons, causing β-lobe fusion. This is more common in females than in males. Homozygous mutant clones in the subesophageal ganglion (SOG) exhibit striking misprojection phenotypes. The normal elaboration and symmetry of arbors in mid-pupae is diminished; ventral dendritic branches do not show normal density, and anterior projections wander and fail to extend. Neurons also fail to fasciculate normally. In wild-type the SOG midline-crossing tract contains tightly bundled projections. In mutant clones, projections stray from this tract, apparently losing some adherent ability. Photoreceptor neurons also mis-project. In mutant clones R1-R6 cells overshoot the lamina, and a number now target the medulla.