Many cells in the body arise during embryogenesis and later from self-renewing stem cells. It works like this diagram in which the top cell, colored red, is a stem cell capable of dividing and producing another stem cell and what I have called a terminal cell, colored green:
 

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The terminal cell may never divide again or it may be capable of producing just a few generations of descendants. In your body there are many stem cells that produce blood components, skin, muscle and other cell types (the green cells in the diagram). In some cases, these stem cells are active throughout life. In other cases, they may lie dormant, only to be awakened under special emergency circumstances to try to heal an injury.

In my view, many cancers are caused by stem cells that are supposed to quit dividing but have not, or by stem cells that have lain dormant for a long time but have been reactivated by something, say a so-called carcinogen, or have been altered genetically (mutated) so that they don’t stop dividing when they’re supposed to. What this means is that most cells in a tumor (neoplasm) are the "terminal" cells while a few are the stem cells responsible for the continued gowth and metastasis of the cancer. The terminal cells may look crappy under the microscope (and this gives rise to all sorts of "scores" supposedly related to the aggressiveness to the tumor) but the real nasty guys are the stem cells which may be resistant to chemo.

This situation is the subject of a very exciting news report from Reuters:

Studies in animals and women with advanced breast cancer showed the experimental compound MK-0752, under development by Merck & Co Inc, was able to kill off cancer stem cells that linger in the breast after chemotherapy.

Researchers are still trying to understand the role cancer stem cells play in promoting different types of cancer, but many teams think they may explain why so many cancers come back even after treatment with powerful chemotherapy and radiation.

“These cells are different from the tumor. They are resistant to therapy. They regrow. They cause relapse and metastases,” said Dr. Jenny Chang of Baylor College of Medicine, who presented her findings at the American Association for Cancer Research San Antonio Breast Cancer Symposium.

A team at the Massachusetts Institute of Technology and the Broad Institute in August reported a similar finding in breast cancer cells and in mice in the journal Cell.

This is tremendously exciting because the way to kill off cancer is to destroy the cancerous stem cells. If this finding holds up and other drugs are discovered that can selectively kill cancerous stem cells, it will revolutionize cancer treatment.

Stay tuned. The press mostly won’t understand this, but I think it’s the most exciting thing in cancer treatment in many years. Oh, and the drug is from one of those very nasty companies, Merck, who doesn’t care about you but only about making money. It’s just an accidental and surprising side effect that their research may save your life. Sure.