SPDL1 and hepatocellular carcinoma: Exploring more relevant serum proteins, such as sPD-1, sCTLA-4, sPD-L1, sTIM-3, sBTLA, sVISTA, and sLAG-3, in addition to examining their gene expression levels in PBMCs, may help identify effective serum markers for predicting the survival outcomes of HCC patients [31,32].