CA9 and cancer: To effectively combat hypoxic and acidic cellular stress, cancer cells activate a network of enzymes and transporters that function to maintain pHi homeostasis (Parks et al., 2013), including the tumour-associated carbonic anhydrases (CA) IX (CAIX) and CAXII (Neri and Supuran, 2011; Corbet and Feron, 2017).