PRKN and Parkinson disease: However, unlike parkin cKO mice, which show neurodegeneration at later stages, and also unlike PD- and neurotoxin-based models (Rieker et al., 2011; Garcia-Esparcia et al., 2015), RNA polymerase I activity is not severely impaired in hA53T-SNCA/PINK1KO, PINK1 KO or DJ-1/PINK1 DKO mice, supporting the hypothesis that compensatory responses might sustain nucleolar activity (Mullin and Schapira, 2015).