Scientists have also been trying to develop vaccines against existing cancers. Some researchers believe that cancer cells routinely arise and are destroyed by the healthy immune system; cancer forms when the immune system fails to destroy them. They are separating proteins from cancer cells and immunizing cancer patients against those proteins, in the hope of stimulating an immune reaction that would kill the cancer cells. Therapeutic cancer vaccines are being developed for the treatment of breast, lung, colon, skin, kidney, prostate, and other cancers. But they have not yet been proven to work in phase 3 human trials, and have not been approved by the U.S. Food and Drug Administration or by European Union regulatory agencies.