The identification of so-called checkpoints in T-cell immunity—namely, the molecules programmed death-1 (PD-1) and cytotoxic T-lymphocyte-associated protein 4 (CTLA-4), as well as the development of function-blocking antibodies against these molecules, have paved the way towards our understanding of the relevance of the immune system against cancer and its manipulation. The gene discussed is CTLA4; the disease is cancer.