CA9 and cancer: In addition, among other important gene products modulated by HIF-1α, monocarboxylate transporter-4 (MCT-4) and carbonic anhydrase IX (CAIX) are often overexpressed in several cancer types as compared with normal tissues and involved in the pH regulatory system of hypoxic cancer cells characterized by a high rate of glycolytic metabolism and production of acidic metabolites such as lactic and carbonic acids (Fig. 3) [113–116].