CA9 and cancer: CA9 expression has been associated with tumor acid–base homeostasis under conditions of hypoxia, cancer progression, metastasis, and impaired response to traditional therapy [4, 5, 9] and is found in many types of cancer, including those of the kidney, bile duct, stomach, esophagus, lung, breast, cervix, ovaries, bladder, brain, head and neck, and oral cavity [4–15].