While high-dose interleukin-2 (IL-2) shrinks about 15% of melanomas, this U.S. Food and Drug Administration (FDA)-approved immunotherapy comes at the high cost of seizures and other major side effects. Now, a new study suggests there may be a way to tell when people with melanoma are benefiting from IL-2, sparing those who are not from unnecessarily enduring the downside of this treatment. The researchers found that when people with melanoma were treated with high-dose IL-2, those who did not benefit also had high levels of a particular type of white blood cell. These cells are regulatory T cells that produce a protein called ICOS (inducible T cell costimulator), which is linked to suppression of the immune system.