CTLA4 and neoplasm: The mechanism of action of anti-CTLA4 has been associated with its ability to deplete regulatory T cells in the tumor [19], and depletion of regulatory T cells concurrent or post-RT has been shown to improve tumor control by radiation therapy.[21, 22] Our data demonstrate that radiation followed by anti-CTLA4 blockade did improve radiation efficacy, but not to the same degree as pretreatment and that pretreatment depletion of regulatory T cells could also improve responses to radiation.