SPHK1 and breast cancer: A toxic phthalate, benzyl butyl phthalate (BBP), activated aryl hydrocarbon receptor in breast cancer cells to stimulate sphingosine kinase 1 (SPHK1)/sphingosine 1-phosphate (S1P)/sphingosine-1-phosphate receptor 3 (S1PR3) signaling and enhance formation of metastasis-initiating BCSCs.