SPP1 and esophageal squamous cell carcinoma: While high plasma OPN level has been associated with metastatic features in a variety of cancers such as gastric cancer [31], hepatocellular carcinoma, esophageal squamous cell carcinoma [32], renal cell carcinoma [33], prostate cancer [34] and breast cancer patients [35], its correlation with CRC metastasis has not been demonstrated thus far.