CREB1 and major depressive disorder: More importantly, the overexpression of mmu_circ_0001223 in PC12 cells by lentiviral system resulted into remarkable increase of CREB1 and BNDF expression, showing that the implication of mmu_circ_0001223 in depression might be mediated by the modulation of CREB1 and BNDF‐related signaling pathways.