ew 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.

Truly 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.