This observation is especially interesting in the context of recent phase II clinical trial data showing that the antihyperglycaemic drug exenatide (a glucagon-like peptide-1 agonist) had efficacy in reducing off-medication motor symptoms in PD.44 Potentially shared cellular signalling pathways for this group of drugs and PD pathophysiology have also been highlighted.45 It is conceivable that our results may therefore reflect confounding by drug treatment—that is, if the treatment of diabetes differs systematically between individuals at high and low risk of PD. This evidence concerns the gene GCG and diabetes mellitus.