In PD studies, SH-SY5Y cells are widely used as an in vitro model to mimic impaired dopamine homeostasis, and to elucidate the mechanism(s) underlying 1-Methyl-4-phenylpyridinium (MPP+)-induced neurotoxicity, as these cells express both dopamine receptors and the dopamine transporter (DAT) [MPP+ is the toxic metabolite of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and a neurotoxin used to induce a PD phenotype] [99]. The gene discussed is SLC6A3; the disease is Parkinson disease.