CA9 and neoplasm: The development of small molecule inhibitors of CAIX has also been intensely pursued in recent years, aided by an increased understanding of the biology of CAIX, which has revealed that hypoxia induced CAIX is strongly expressed on cancer stem cells (CSC), and inhibiting CAIX leads to depletion of such cells, resulting in suppression of tumor progression and metastasis [19,20,21,22].