PI31^KO homozygous larvae have similar overall neuromuscular junction structure, but significantly reduced presynaptic active zone assembly, compared to controls; motor neurons also have significantly reduced axonal transport of proteasomes, compared to controls; homozygous mutant clones (generated using hs-FLP during embryonic stage) have a significant cell-autonomous reduction in presynaptic active zone assembly at the neuromuscular junction, compared to controls.

Mutants die at the late third instar/early pupal stage. Larvae develop melanotic tumors, are slower to develop and are smaller than wild type.

Expression of PI31^{Scer\UAS.T:Ivir\HA1} using Scer\GAL4^tub.PU and Scer\GAL80^ts.αTub84B at 23[o]C in a PI31^KO background results in viable 'mosaic' adults that lack PI31 function specifically in the germ cells. The testes of these flies are irregular and display a meiotic arrest phenotype, characterized by an abundance of underdeveloped 16-cell stage cysts. Mitosis of germline cells proceeds normally, but meiosis does not. Germ cell differentiation is prevented and a progenitor identity persists.