CA9 and neoplasm: The absence of detectable differences between the pHi values of the CA9 tumours and the EV tumours (7.00 ± 0.04 vs 6.99 ± 0.04; mean ± SEM) (Fig. 6a) suggests that exofacial CAIX expression has little net effect on tumour pHi in vivo and that the effect of CAIX expression is primarily on tumour pHe. This might be due to the weaker buffering capacity and smaller volume of the extracellular space compared with the intracellular space, both of which would tend to amplify acid-load induced pHe changes.