With respect to peripheral markers of ketamine treatment response, one previous study demonstrated that higher serum levels of the synaptic protein Shank3 correlated with improvement following ketamine administration in bipolar depression46; a second study found that lower levels of adiponectin, an anti-inflammatory protein, correlated with treatment response in MDD and bipolar depression.47 In our study, we found that low levels of FGF-2 (<34 pg ml−1) were predictive of ketamine treatment response (92% specific; Figure 3). Here, SHANK3 is linked to major depressive disorder.