CA IX is one of three exofacial CA isoforms, along with CA IV and CA XII, and has a unique position in this enzyme family due to its strong association with cancer, hypoxia-related expression pattern, acidic pKa optimum, and inclusion of an extra proteoglycan-like domain protruding from the globular catalytic domain of the enzyme, which is anchored in the plasma membrane via a single-pass transmembrane region and a short cytoplasmic tail [23–25]. This evidence concerns the gene CA9 and cancer.