GRM4 and Parkinson disease: It is differentially expressed in depressed individuals and has been shown to target the Metabotropic Glutamate Receptor 4 (GRM4; Lindsley and Hopkins, 2012), a synaptic molecule modulating neurotransmission that also constitutes an attractive therapeutic target for Parkinson’s disease and schizophrenia (Figure 2; Lopez et al., 2014).