Most cells in the body have a finite lifespan and eventually die, a process known as programmed cell death or apoptosis. Cancer cells, on the other hand, do not commit suicide and thus continue to divide and create new cells, leading to the development of a tumor mass as seen in breast, lung prostate cancers, for example, or over production e.g. of white blood cells in leukemia.

When chemotherapy or radiotherapy is administered, the goal is often to try and induce apoptosis, thereby leading to tumor shrinkage.