CA9 and cancer: It has already been reported that (i) hypoxic conditions associated with tumours are related to resistance to chemotherapy and radiotherapy (Gray et al, 1953; Brizel et al, 1996; Hockel et al, 1996); (ii) low extracellular pH is a typical feature of the tumour microenvironment, which has an impact on cancer development (Svastová et al, 2004); and (iii) CA9 contributes to tumour growth and survival (Robertson et al, 2004).