The goal of cancer treatment is to kill cancer cells while not hurting the body’s healthy cells. One way this is done is by using a drug to target a specific genetic change (also referred to as a mutation) in the cancer cells that is not seen in healthy cells. Historically, many of these mutations have been associated with an aggressive biology and sometimes resistance to cancer treatments, such as chemotherapy. But with immunotherapy, which is treatment given to stimulate or remove inhibition of the immune system to help combat a cancer, a higher number of mutations in a tumor can also be associated with a greater probability of response to treatment. This is because the immune system has a better opportunity to attack the cancer effectively if it can recognize it, and the body’s immune system may better identify cancer cells that have a higher number of mutations.