Immune cells infiltrate tumors and make up a significant component of the multicellular cancer microenvironment, yet the immune system often fails to prevent tumor formation and progression. One explanation for this paradox is the presence of tolerance-promoting regulatory T cells (Tregs). Tregs were known to be essential for maintaining self-tolerance. Tregs have been found to promote tolerance to tumors in mouse models. Treg infiltration in human tumors and malignant ascites is associated with worse clinical outcomes for various types of cancers.