ABCG2 and neoplasm: The S1P generated intracellularly is exported into the extracellular space leading to inside-out signaling in the tumor microenvironment via specific S1P transporters including protein spinster homologue 2 (SPNS2), major facilitator superfamily d2b (Mfsd2b), ATP-binding cassette sub-family C member 1 (ABCC1), and ATP-binding cassette sub-family G member 2 (ABCG2) (Figure 4A).