GPR17 and Down syndrome: Overall, despite the presence of pyrimidine signaling molecules acting on the P2Y2, 4, 6, 14 receptor subtypes (von Kügelgen and Hoffmann, 2016) and on the P2Y-like receptor GPR17 (see section Involvement of the Purinergic System in Brain Alterations Observed in Down Syndrome), this system is referred to as the “purinergic system” (Burnstock, 2017).