It is also possible that active movement of S1P from inside to outside the cells (Fig 1A) through S1P transporters (SPNS2, spinster homolog 2; ABCC1, ATP-binding cassette sub-family C member 1; ABCG2, ATP-binding cassette sub-family G member 2) [32] might reduce S1P levels in HCC tissues. This evidence concerns the gene ABCG2 and hepatocellular carcinoma.