abnormal cells evade the human body's immune system, and of those that do most
delete themselves through apoptosis-programmed cell death. Even when an abnormal
cell persists, it usually doesn't replicate uncontrollably.
cancerous cells know no such restraints. They can multiply and exist indefinitely
outside their normal tissue environment. Breast cancer cells, for example, can
break through the basement membrane that ordinarily segregates mammary duct
cells from supportive tissue, then spread throughout the body.