We found that serum IGF-2 levels were much lower in both the mania and depression subgroups than those in healthy controls (mania: 67.19 ± 21.52 ng/ml, depression: 63.43 ± 20.67 ng/ml, controls: 88.72 ± 31.55 ng/ml; H = 29.692, P < 0.001) (Figure 1). This evidence concerns the gene IGF2 and major depressive disorder.