Usp5⁹ mutant eye clones (generated using ey-FLP mediated mitotic recombination) are rough, small and have disordered bristles. Some of the ommatidia fail to develop.

Usp5⁹ mutant eye clones exhibit photoreceptor differentiation defects. The morphogenetic furrow is delayed and there are fewer R8 photoreceptors in Usp5⁹ mutant eyes (generated in a minute background) than in controls, but the remaining R8 photoreceptors show outgrowth. Most cone cells fail to differentiate in the Usp5⁹ mutant clones.

There is an increase in apoptotic cells in Usp5⁹ mutant eye disc clones (generated in a minute background), especially anterior to the morphogenetic furrow.

Blocking apoptosis suppresses the photoreceptor differentiation phenotype seen in Usp5⁹ mutant eye disc clones.