T cell immunotherapy uses the immune system to kill cancer cells. To increase the efficacy of this new treatment, researchers aimed to discover a master regulator of T cell fate. The signaling lipid sphingosine 1-phosphate (S1P) influences the T cell lineage: high levels of S1P lead to an inhibitory regulatory T cell phenotype, while low levels of S1P lead to a central memory-like phenotype, which exhibit increased anti-cancer functions.