In our model of comorbid depression and PD, genotype (F1,25 = 4.80; p < 0.05), treatment (F3,25 = 19.76; p < 0.001) and their interaction (F3,25 = 3.14; p < 0.05) had significant effects on c-fos expression. The gene discussed is FOS; the disease is major depressive disorder.