While this compensation may initially be helpful, the terminal DA transporter and presynaptic D2 receptor is virtually absent in later stage PD patients and since 5-HT cells lack autoregulatory mechanisms for DA, its unregulated release may manifest as unwanted side effects such as LID and PDAP (Calabresi et al., 2015; Yoo et al., 2018; Sellnow et al., 2019). The gene discussed is SLC6A3; the disease is Parkinson disease.