FOS and major depressive disorder: The first module involves a cascade reaction through ADCYAP1-ADCYAP2R3-GNAS-ADCY3-cAMP-PRKACA, controlling downstream genes like GRIA2, GRIN8A, GSK1A, CREB1, BDNF, FOS, ATF1, MAPK1, and JUND, which play a role in treating depression.