CA9 and neoplasm: We then confirmed that MDA-MB-231 tumours also exhibit higher levels of hypoxia, by assessing tumour expression of Carbon Anhydrase IX (CA-IX), a protein upregulated in hypoxic conditions (Fig. 4b; 0.53 ± 0.04 MCF-7 vs. 0.73 ± 0.02 MDA-MB-231).