Tissue based biomarkersTechnique: Assessing the expression of proteins induced in hypoxic conditions (GLUT1, CA9)Benefits: The location of protein expression and data on functional status can be provided through IHC. Gives an estimate of hypoxia heterogeneity within different cancer regionsDrawbacks: Indirect measurement of O2 concentrations. Invasive procedure (biopsy required). Protein expression can be manipulated by factors other than hypoxia. Method limited to a small number of biomarkers due to antibody sensitivity and specificity issues. Real-time measurements are not possible. Here, CA9 is linked to cancer.