SPHK1 and breast carcinoma: Multiple clinical observations have subsequently reported the contribution of high SPHK1 expression to therapy resistance, disease recurrence and metastasis in breast cancer patients (Long et al., 2010a; Watson et al., 2010; Ohotski et al., 2012; Do et al., 2017; Acharya et al., 2019), all of which are similar to the clinical implications of breast CSCs.