Fragile X syndrome (FXS) is characterized by learning disabilities and behavioural problems and is linked to dysregulated mRNA translation. Here, the authors treated a mouse model of FXS with the type 2 diabetes drug metformin, which, among other effects, suppresses translation. Metformin treatment rescued various phenotypes in these mice and downregulated translation-linked signalling pathways, decreasing the expression of matrix metalloproteinase 9, which is implicated in FXS. This suggests that metformin may have therapeutic potential for this disorder.