A type of immunotherapy drug that is used to treat advanced kidney cancer. Checkpoint inhibitors block certain proteins (checkpoint proteins) made by some types of cells in the immune system, such as T cells, and some cancer cells. Checkpoint proteins keep the immune response in check, and can prevent T cells from killing cancer cells. When checkpoint inhibitors block checkpoint proteins, the ‘brakes’ on the immune system are released, and T cells are able to kill cancer cells more effectively. Examples of checkpoint proteins include PD-1/PD-L1 and CTLA-4.