In previous studies, it was demonstrated that SphK1, but not SphK2, is involved in S1P export from BC cells through the action of ABCC1 and ABCG2, which plays a part of signal transduction for the non-genomic action of estradiol and tamoxifen resistance in BC (Figure 2A–C) [27,28,29]. The gene discussed is ABCG2; the disease is breast cancer.