FGF1 and major depressive disorder: In addition, the 5Htt +/− genotype reduced the expression of fibroblast growth factor 1 (Fgf1), the major ligand of Fgfr1. Interestingly, a dysfunction in FGF signaling has been suggested to play an important role in the etiology of mood disorders [50], which is underscored by the finding that patients with major depression disorder (MDD) show an increased FGFR1 expression in various hippocampal subregions [51].