Many scientists consider embryonic stem cells to be ideal to treat disease because they multiply extensively and can differentiate into all the cells and tissues of the body. However, to obtain them, five-day-old embryos have to be destroyed.

To avoid the ethical and political hurdles that surround stem cells taken from embryos, scientists are hunting for alternative sources.

Until recently, scientists thought it impossible that these bone marrow cells could 'go back in time' and re-invent themselves to produce completely different cell types such as brain, nerve, gut or skin. However, scientists in the United States have recently identified a stem cell from adult bone marrow that they believe can grow into any other cell type. "It's an extraordinary thing," says stem-cell expert Austin Smith of the Centre for Genome Research in Edinburgh, UK.

Not only would stem cells taken from a consenting adult be ethically acceptable to most people and governments, they would also be better for patients. Imagine you had a disease killing your brain cells. Stem cells could be taken from your bone marrow, manipulated in the laboratory so that they become brain cells, and implanted back into your brain - hence no rejection of the transplant by your immune system.

This is a fantastically exciting prospect - if it works. Early results look promising, but scientists don't know how versatile stem cells from bone marrow really are. They are far more confident of what stem cells from embryos can do.

Ultimately, different stem cell types might best treat different diseases, so most scientists would opt for continued research on both types.

A final option for a source of stem cells is umbilical cord blood - usually discarded at birth. For some time now, mothers have been able to donate the stem cells within their baby’s umbilical cord to public ‘cord blood banks’. Cord blood collected in this way has been used to successfully treat leukaemia and immune disorders in unrelated patients. Now companies have begun offering a service to collect and store a mother’s placental blood for a fee. This blood could be used to treat the baby if it were ever ill. The companies claim that in the future, cord blood could provide a source of stem cells to cure stroke, heart disease, diabetes, Parkinson's disease and Muscular Dystrophy. In addition, at the moment these stem cells could be used to treat blood disorders such as leukaemia. This procedure raises issues such as, perhaps only the rich will be able to store their cells, and that in some disorders the cord stem cells will also be affected and therefore no good to transplant.

The beauty of collecting these stem cells is that they are taken without affecting the mother or child. They are also 100% compatible with the baby should he or she ever develop an illness and be in need of stem cells.

These companies argue that in addition, the cord blood from the baby might provide a source of compatible stem cells for the baby's relations - brothers and sisters, parents and grandparents.

For a few thousand dollars - could you afford not to store them?

• What are stem cells?
• Where do the embryos come from?
• Other sources of human stem cells
• Human cloning
• Human cloning - the risks