Ipilimumab, a cytotoxic T lymphocyte-associated antigen 4 (CTLA4)-blocking antibody, can induce clinical responses in cancer patients, but it can also induce some considerable and even fatal side effects. Hypophysitis (inflammation of the pituitary gland) is a side effect that occurs in a small proportion of patients treated with ipilimumab. Using a mouse model of ipilimumab-induced hypophysitis, Iwama and colleagues found that CTLA4 was expressed in certain cells within pituitary glands and these were the site of inflammation.