Sphingosine 1-phosphate (S1P) is a biologically active lysophospholipid that transmits signals through a family of five G-protein-coupled receptors to regulate cell proliferation, migration, cytoskeletal organization, and differentiation. S1P1 was the first identified S1P receptor. It primarily couples to PTX-sensitive Gi/o proteins and mediates S1P-induced adenylate cyclase inhibition. Expression of S1P1 is pervasive, including spleen, brain, heart, lung, adipose tissues, liver, thymus, kidney, and skeletal muscle. The deletion of S1P1 in mice results in embryonic lethality with death attributable to incomplete vascular maturation. Recent reports demonstrate specific roles for S1P1 in lymphocyte recirculation/egress.