The current available explanations for tumor specificity of 3-BrPA are attributed to three aspects [40]: (1) specific overexpression of mitochondria-bound HK-II and high rate of glycolysis in tumor cells; (2) tumor cells selectively overexpress monocarboxylate transporters (MCTs), a family of transmembrane transporters, which are known to export the excess lactate excreted by tumor cells to avoid the intracellular acidification and cell death; and (3) acidic extracellular microenvironment of tumor tissues. The gene discussed is HK2; the disease is neoplasm.