Increased intracellular cAMP/cGMP may suppress tumour growth and hence be a protective mechanism against tumour progression.3 PDE inhibitors that limit cAMP or cGMP hydrolysis have thus been identified as possible anticancer medicines (Fig. 34).125 However, some cancers, such as adenocarcinoma, can stimulate cell proliferation when cAMP levels are elevated.127,128. Here, ALDH7A1 is linked to neoplasm.