SPHK1 and breast carcinoma: However, in MDA-MB-231 human breast cancer cell lines, SphK1 expression was not observed in this study in cells with approximately 0.5 kPa stiffness, which is similar to the rigidity of normal breast tissue, and was profoundly increased in cells with approximately 2.5 kPa stiffness, corresponding to breast cancer stiffness11.