It is worth notice, that these PET studies were performed in FXS adult patients between 18 and 60 years old, which may explain the unsuccessful results in mGluR5 antagonist clinical trials that have addressed a similar age range (Bailey et al., 2015; Berry-Kravis et al., 2016; Youssef et al., 2018). Here, GRM5 is linked to fragile X syndrome.