PDE5 is differentially expressed in every subtype of breast cancer cells, including luminal A (ER-positive/PR + HER2 negative), luminal B (ER + and/or PR + HER2+), HER2-enriched (ER- and PR-/HER2+), and triple-negative (i.e., the lack of all three receptors). This evidence concerns the gene ERBB2 and breast cancer.