Two prospective clinical trials (2013-2017) for gaining approval in the European Union have shown the significance of the presence of aerotolerant anaerobic cancer cells for clinical outcome and response to therapy. Leukemia patients with refractory disease (not responding to standard chemoimmunotherapy (sCIT)) had a significant amount of aerotolerant anaerobic leukemia cells in their PBMC fraction whereas those responding well to sCIT did not (median PFS 216 days vs not reached, p=0.008; multivariate analysis for shorter TFS: hazard ratio 2.37, p=0.011). Fig. 3 illustrates that colon cancer patients after curative resection of their tumor display a very short disease free survival when having a detectable amount of aerotolerant anaerobic cancer cells in their primary tumor samples. Disease free survival (DFS) measured from the day of sample acquisition was significantly shorter (UICC I-IV: median DFS 6.8 months vs not reached, p=0.000; UICC I-III: median DFS 17.2 months vs not reached, p=0.004) for patients with cancer cells preferably growing under anoxia (aerotolerant anaerobes). Multivariate analysis identified the presence of (aerotolerant) anaerobic cancer cells as the strongest independent risk factor for shorter DFS (hazard ratio 3.92, p=0.004).