The function of CA9 (carbonic anhydrase 9, ENSG00000107159) is as follows. Catalyzes the interconversion between carbon dioxide and water and the dissociated ions of carbonic acid (i.e. bicarbonate and hydrogen ions). {ECO:0000269|PubMed:17314045, ECO:0000269|PubMed:17705204, ECO:0000269|PubMed:18703501, ECO:0000269|PubMed:19186056, ECO:0000269|PubMed:19206230, ECO:0000269|PubMed:19805286}.