Cells from the immune system called CD4+ regulatory T cells, or simply Treg, are linked to tumor prognosis: the more Treg cells present in a tumor, the worst the prognosis. We know from previous research studies that Treg cells can infiltrate into the tumor microenvironment. There, they inhibit anti-tumor immunity, which is the self-defense mechanism that the body puts in place to fight cancer cells, stimulating tumor growth. However, how tumor-infiltrating Treg cells are activated and regulated was, until this study, unknown. A better understanding of how Treg cells contribute to tumor growth is the first step in designing novel therapies for melanoma and many other cancers.