SPP1 and head and neck cancer: The advantage of OPN lies in its easy detection in plasma or serum of cancer patients [11] and in its probable relation to hypoxic radiation resistance: In radiotherapy of head-and-neck cancer, pre-treatment OPN plasma levels were able to successfully predict tumor hypoxia and to identify patients who benefitted from treatment with the hypoxic radiosensitizer nimorazole [12, 13].