In the pathogenesis and progression of PD and DM, insulin and dopamine are regulated mutually, and hypoinsulinemia induced by streptozotocin can reduce the levels of dopamine transporter (DAT) and tyrosine hydroxylase (TH) transcription in the substantia nigra pars compacta (Lima et al., 2014). The gene discussed is SLC6A3; the disease is diabetes mellitus.