FTY720 is a derivative of ISP-1 (myriocin), a fungal metabolite of the Chinese herb Iscaria sinclarii as well as a structural analog of sphingosine. It is a novel immune modulator that prolongs allograft transplant survival in numerous models by inhibiting lymphocyte emigration from lymphoid organs. FTY720 is phosphorylated by sphingosine kinase, which then acts as a potent agonist at four of the sphingosine-1-phosphate (S1P) receptors (S1P1, S1P3, S1P4, and S1P5). Down-regulation of S1P1 receptors on T and B lymphocytes by FTY720 results in defective egress of these cells from spleen, lymph nodes, and Peyer's patch. azido-FTY720 is a highly photoreactive analog of FTY720 that can be used to identify receptor binding sites for this ligand.