In the PINK1 Pathogenic mutations in the kinase PINK1 are causally related to Parkinson's disease (PD). One hypothesis proposes that PINK1 regulates mitophagy—the clearance of dysfunctional mitochondria. A second hypothesis suggests that PINK1 has a direct effect on mitochondrial complex I, affecting the maintenance of the electron transport chain (ETC) resulting in decreased mitochondrial membrane potential and dysfunctional mitochondria. In support of the second hypothesis, Morais et al. (p. 203 , published online 20 March) observed a complex I deficit in fibroblasts and neurons derived from induced pluripotent stem cells from PINK1 patients before any mitophagy was induced. The phosphoproteome of complex I in liver and brain from mice deficient for Pink1, compared to wild-type animals, revealed that Ser ²⁵⁰ in complex I subunit NdufA10 was differentially phosphorylated. Ser ²⁵⁰ is critically involved in the reduction of ubiquinone by complex I, explaining why Pink1 knockout mice, flies, and patient cell lines show decreased mitochondrial membrane potential. Synaptic defects in pink1 null mutant Drosophila could be rescued using phosphomimetic NdufA10.