PRKN and Parkinson disease: Each of the two transgenes was also expressed upon crossing to an original parkin E3 ubiquitin protein ligase (Prkn) constitutive KO strain (MGI allele symbol: Prkn<tm1Ykt>), resulting in the manifestation of PD-associated phenotypic traits, with distinctive death of substantia nigra dopaminergic neurons, following increased endoplasmic reticulum stress and dopamine toxicity [19].