CA9 and carcinoma: These findings are in agreement with previous results in carcinoma cells exposed to severe hypoxic conditions.14 When the tissues positive for CAIX staining were grouped into high- and low expression according to the scoring system reported in Supplementary Table 2, we observed that high expression levels of CAIX were associated with low expression of PCFT (P = 0.016, Fig. 1a), which was assessed as described previously.18 These results are in agreement with the role of this folate/antifolate transporter in cells with acidic pH,33 which commonly characterises the TME.