SLC9A1 and neoplasm: The NHE1 is specifically involved in cellular acid–base balance and is the predominant isoform expressed in tumors, where it has been shown that it contributes to cellular pH homeostasis, cell transformation, proliferation, motility, migration, tumor growth, invasion, activation of the metastatic process, resistance to chemotherapy and probably also to the spontaneous regression of cancer [4,31,37,82-84].