Crossing Parkin homozygous mutant mice with ‘mutator mice’ (Box 4) elicits the PD phenotype with degeneration of dopaminergic neurons in the substantia nigra and motor coordination defects, which can be completely reverted by treating the mice with l-3,4-dihydroxyphenylalanine (L-DOPA) (Pickrell et al., 2015). The gene discussed is PRKN; the disease is Parkinson disease.