In the neurons derived from PD patient iPSCs carrying α-synuclein A53T mutation, Chung et al. found that the hallmark of ER stress, PDI increased, but the marker of ER stress-induced cell death, CHOP, did not have the change (Chung et al., 2013), revealing that these mutant neurons are still at an early pathologic stage. Here, P4HB is linked to Parkinson disease.