Moreover, expression of IGF1R and SphK1/SIP-receptors has been shown to contribute to tamoxifen resistance in ER-positive breast cancer [16, 25, 26] which further highlights the need to better understand the significance of IGF1R and SphK1 co-expression and their contribution to anti-estrogen therapy resistance in breast cancer. This evidence concerns the gene SPHK1 and breast cancer.