To investigate whether the SPHK-S1P axis is altered in breast CSCs, we evaluated the basal expression levels of SPHK1, phosphorylated SPHK1, and SPHK2 in a panel of breast CSCs derived from MCF-7, SKBR3, MDA-MB-468, and HCC38 breast cancer cells. Here, SPHK1 is linked to breast cancer.