SPHK1 and cancer: Several growth factors and steroid hormones, such as TGF-ß and 17ß-estradiol could be related to up-regulation of Sphk1 in cancer cells, producing high amounts of SP, which can bind extracellularly to five specific G-protein-coupled receptors, located on the cell surface (named S1P1, 2, 3, 4, 5) or intracellularly stimulate cell survival [3, 4].