On the other hand, it is striking that we observe reduced mGluR5 binding specifically in the hippocampus and the amygdala regions typically involved early in Alzheimer's disease (Zanchi, Giannakopoulos, Borgwardt, Rodriguez, & Haller, 2017)—but not in other brain regions where we would expect synaptic loss in AD patients (Scheff, Neltner, & Nelson, 2014). Here, GRM5 is linked to early-onset autosomal dominant Alzheimer disease.