PDE5A and breast cancer: PDE5 contributes to the regulation of intracellular cyclic GMP (cGMP) pools (see Figure 1) that have been shown to be decreased along with protein kinase-G (PKG), a downstream effector of cGMP, in variety of different tumors such as breast cancer, colon cancer and human oral squamous cell carcinoma (hOSCC) (Spoto et al., 2003; Zhu and Strada, 2007; Di et al., 2010).