A new study suggests that damaged mitochondrial DNA (mtDNA) can trigger and aid the spread of Parkinson disease (PD)-like pathology in the brain. Evidence of mtDNA deletions affecting complex I respiratory chain subunits was found in post-mortem medial frontal gyrus tissue from patients with sporadic PD (sPD). Similar mtDNA damage was observed in a mouse model of sPD with dementia (sPDD). The damaged mtDNA was released from neurons and spread to distant brain regions, where it exerted neurotoxicity. Moreover, when injected into the brains of healthy mice, this mtDNA induced sPDD-like pathology and behavioural symptoms.