Moreover, compared to other CA isoforms, CAIX appears to be more sensitive to pH changes near the pK, with a Hill cooperativity number of ~2; these properties suit it ideally to the role of an acidic pH-stat.12,17 In contrast, CAXII (another exofacial cancer-related isoform) is active across the pH range and would not act as a pH-stat, since the persistence of CAXII activity at low pHe, means that the reaction will continue unopposed.45 Here, CA9 is linked to cancer.