Finally, we demonstrate that the rescue of the olfactory performance in PD mutants is specific for OPN, because expressing wild-type CDS in another affected cell type, ‘T1-interneurons of the visual system’, of knock-in PD animals (we chose to test this in Rab39BG192R mutants) does not improve olfactory performance (Figure 5c and d; ‘T1>hRab39B +’, red label). The gene discussed is SPP1; the disease is Parkinson disease.