Phosphodiesterase 5 (PDE5) is highly expressed in a variety of malignant tumors, such as bladder cancer, breast cancer, non-small cell lung cancer, and colorectal cancer, indicating that PDE5 plays an important role in tumorigenesis and that inhibition of PDE5 activity may have antitumor effects [10, 11]. This evidence concerns the gene PDE5A and breast cancer.