Intracranial delivery of RNA-guided endonucleases Cas9 and Cpf1 to adult mouse brains by CRISPR-Gold and targeted knockout of the metabotropic glutamate receptor 5 (mGluR5) gene can effectively reduce mGluR5 levels and reverse the repetitive behaviors of autism caused by fragile X syndrome (Lee et al., 2018). Here, GRM5 is linked to fragile X syndrome.