According to the expression levels of the lactate transporters (MCT1 and MCT4) or the hypoxic biomarkers (HIF-1α and CAIX) in cancer cells, tumors were classified as hypoxic (Hx+) when over-expressed HIF-1α and/or CAIX regardless of the levels of MCT1 or MCT4 (142/245, 58%), and lactate-transporter tumors (LAc+) when overexpressed MCT1 and/or MCT4 regardless of the levels of HIF-1α or CAIX (51/75, 68%) (Table 4). The gene discussed is CA9; the disease is cancer.