SPP1 and neoplasm: To the authors' knowledge, the ELISA system utilized in the two reports on plasma OPN as a marker of tumor hypoxia in head and neck cancer [1,2], is no longer commercially available We therefore performed a comparative analysis, using different commercial ELISA systems, of plasma OPN levels in 88 archival samples obtained from patients with head and neck cancer or cervix cancer at different timepoints before, during or after radiotherapy.