Further investigation into the role of SK1/S1P in the tumor microenvironment, a key player in both tumor initiation and breast cancer progression, as well as evaluation of Rac signaling in lung endothelial cells, from MMTV-PyMT SK1-/- mice is important to understand the role of SK1 in spontaneous breast tumor initiation and breast cancer metastasis in the MMTV-PyMT mouse model. Here, SPHK1 is linked to breast cancer.