P4HB and Parkinson disease: However, in the SNpc of PD patients, ER stress response proteins including BiP, homocysteine-induced endoplasmic reticulum protein (Herp), and protein disulfide isomerase (PDI) were increased and co-localized with α-synuclein (Conn et al., 2004; Slodzinski et al., 2009; Selvaraj et al., 2012), revealing that chronic ER stress occurs in patients’ SNpc.