CA9 and neoplasm: Carbonic anhydrase IX (CAIX) is a transmembrane carbonic anhydrase isoform that plays an important role in the regulation of the pH microenvironment within tumours, thus facilitating their growth,3 and is therefore considered an interesting drug target for antiproliferative therapy.4, 5 CAIX is weakly expressed post‐natally in most tissues except in tumours.