CA9 and breast cancer: Being induced by hypoxia and HIF-1, CAIX overexpression is a sign of poor prognosis and promotes BC invasion and invasion in hypoxic microenvironments [79,80,81,82] Furthermore, CAIX expression, as well as NHE activity, are associated with estrogen receptor negative (ER−) BC tumors and a poor prognosis [11,83,84,85].