SPP1 and neoplasm: The expression of OPN is negatively correlated with the prognosis of patients with colorectal, head, and neck cancers (3, 4), and it has been identified as a biomarker for tumor progression in prostate cancer (5), non-clear cell renal cell carcinoma (6), hepatocellular carcinoma (HCC) (7), and non-small cell lung cancer (NSCLC) (8).