Svenningsson and coworkers examined the levels of NPY, CGRP, and 5-hydroxyindoleacetic acid (5-HIAA), the major serotonin metabolite, in cerebrospinal fluid (CSF) from PD patients, with or without comorbid depression, and compared them to the levels in patients with major depressive disorder. The gene discussed is NPY; the disease is depressive disorder.