SLC6A4 and Parkinson disease: There are several factors increased in PD that could cause SERT dysfunction (⍺-synuclein pathology41, TNF-⍺42,5152, SSRI/SNRI use53), and SERT dysfunction could, in turn, affect plasma serotonin levels; however, the effects of SERT dysfunction on plasma serotonin are nuanced54,55, and the mechanisms connecting SERT to iron remain unclear.