Several P2X7 PET ligands have been described in the literature recently (Figure 3); there are two clinical utilities of P2X7 PET ligands that cross the blood–brain barrier: (a) the PET ligands can be used as a probe to measure central target occupancy of the clinical compound being developed for a CNS indication; (b) since P2X7 activation is related to microglial activation, P2X7 PET ligands can potentially be used as central biomarkers of assessing microglial activation in diseases accompanying neuroinflammation such as mood disorders, schizophrenia, epilepsy, multiple sclerosis, and others. Here, P2RX7 is linked to mood disorder.