In line with the high load of mtDNA deletions in SNc dopaminergic neurons of idiopathic PD patients, parkinsonian features were additionally seen in some patients harboring mutations of POLG (Luoma et al., 2004; Hudson et al., 2007) and TWNK (Suomalainen et al., 1997; Baloh et al., 2007; Kiferle et al., 2013). Here, POLG is linked to Parkinson disease.