PDE5A and neoplasm: The nitric oxide (NO)/cyclic guanosine monophosphate (cGMP) signaling pathway is a major pathway that PDE5 acts principally on, which regulates cell proliferation, tumor development, and tumor progression (Peak et al., 2016).The relationship between the level of cGMP and amino acid metabolism may be a future research direction that clarifies the mechanism of tadalafil treatment in CRC.