ELISA analysis of cGMP with HeLa cells (96-well plate was coated with 1 µg/mL of Goat anti-rabbit IgG (50 µl/well); 0.05 µg/mL of anti-cGMP (50 µl/well) was added) at a concentration of approx. 105 cell/mL (HeLa) or control (lysate buffer only; 25 µl/well). Samples were incubated in monoclonal cGMP antibody (Product # MA5-44557), followed by HRP conjugated cGMP (25 µl/well) and TMB (develop) using a dilution of 1:5,000.