PDE5A and neoplasm: Although several cGMP degrading PDE isozymes have been reported in normal lung and lung tumors, including the cGMP-specific PDE isozyme, PDE5 [10, 18, 22, 23], the critical cGMP PDE isozymes that are induced during lung cancer progression and which are essential for tumor cell proliferation and survival have not been well studied, but may provide useful targets for the development of new anticancer drugs.