To test this hypothesis, we used an in vitro luminescence assay to measure ATP concentration in our neuronal cultures and observed a ~3.2-fold reduction in the intracellular levels of ATP in PARK2 PD hGNs compared to healthy controls (Fig. 5D), as well as a ~1.4-fold decrease in ATP in TDP-43N390D ALS hMNs compared to healthy controls (Fig. 6D). Here, PRKN is linked to Parkinson disease.