Although there is evidence that different OPN isoforms and posttranslational modifications (i.e., phosphorylation, sialation, proteolytic cleavage, transglutaminase crosslinking, and proteolytic processing) may affect different OPN functions [59, 128–130], there is still very scant literature on how tumor-derived OPN may differ from stroma-derived OPN, either structurally or functionally. The gene discussed is SPP1; the disease is neoplasm.