In this study we explored the application of the tracer [111In]In-G250-F(ab’)2 to assess the changes in CAIX expression induced by hypoxic gas breathing (meant to induce hypoxia and hence increase CAIX expression) and inhibition of mitochondrial respiration with atovaquone to reduce hypoxia (and reduce CAIX expression) in human xenografted HNSCC. The gene discussed is CA9; the disease is head and neck squamous cell carcinoma.