SPP1 and hepatocellular carcinoma: Based on the observation that the supplemental provision of rhOPN to OPN KD human HCC cells inhibited apoptosis, and that OPN overexpression in cells rarely affect cell viability and apoptotic cell death in the present study, it may be hypothesized that the anti-apoptotic effect of OPN is due to sOPN rather than iOPN (Figure S3A and SB).