SLC9C1 and neoplasm: For the excretion of lactic acid and protons, malignant tumor cells including MM cells express various types of ion transporters such as H+-ATPase (pump), Na+/H+ exchanger (NHE), and/or monocarboxylate transporters (MCT) (e.g., MCT1, MCT4), which participate in the maintenance of a controlled intracellular pH (Figure 1) [32,37,38,39].