SLC40A1 and Parkinson disease: Furthermore, intracellular iron levels are increased in the substantia nigra region of PD patients [15,16], and iron homeostasis is disrupted in PD patients and animal models due to abnormal expression of either divalent metal transporter 1 (DMT1), an iron importer, or ferroportin 1 (FPN1), an iron exporter [17,18,19].