An analog of sphingosine which, when taken up into cells, is phosphorylated and acts as an agonist at sphingosine-1-phosphate receptors (S1P1/3)[1]. Potently inhibits regulatory T cell proliferation in vitro and in vivo[2]. Enhances microvascular remodeling via expansion of arterioles and capillary networks[3]. Potent immunomodulatory[4]. Clinically effective agent for relapsing forms of multiple sclerosis[5].