FOS and viral infectious disease: Four weeks after the shRNA virus infection, we found a significant increase in the count of c-Fos-positive neurons co-labeled with CamkII-GFP-shRNA-HCN2 compared with the control group (c-Fos+/GFP-CamkII+) (Figure 5A,B; p < 0.001), indicating strong inhibition of vCA1 pyramidal neurons with HCN2 knockdown.