Extraordinary opportunities to study and to treat human diseases are provided by the recently acquired ability to derive stem cells from human embryos. Because these cells form all of the tissues that make up an adult, they afford a chance to study normal human development in the laboratory, to define the abnormalities associated with inherited disease and, in time, perhaps to treat diseases, many of which have no effective treatment at present.

Consider just three situations among many. Cells derived from embryo cells could be used to repair spinal cord injury. It is far from clear exactly what type of cell should be used, how many cells are needed or where they should be placed. Nevertheless, speedy treatment might provide real benefit.

Cells from cloned embryos will reveal the molecular mechanisms that cause inherited diseases such as amyotrophic lateral sclerosis (known as motor neurone disease in some countries). This will allow us to study the disease process in minute detail for the first time and, more important, to screen thousands of compounds that might potentially arrest or even reverse the degeneration.

Finally, genetic diseases may eventually be corrected in children. Imagine a child who has no immune response to infection because of an error in a specific gene. The error could be corrected in cells derived from a cloned embryo, which might then be converted to bone marrow cells that provide the absent immune response. The corrective marrow cells could then be returned to the child.

Clearly, success with embryonic stem cells will depend upon detailed research, and it will take several years, perhaps decades, to bring these ideas to the clinic. Over time, embryo-derived stem cells will revolutionise many aspects of medicine. And yet society hesitates.

In discussing stem cell research, investigators face several critical issues. To some people the idea of producing and using a human embryo is deeply offensive, and these sincerely held views must be recognised. Yet many others do not share these qualms. The early embryo from which stem cells are derived is a ball of cells smaller than a grain of sand. While it has the potential to become a person, it lacks the fundamental human characteristics of being conscious and aware.

An urgent need exists for an informed debate about what we consider to be critical human characteristics, just as there was an equivalent debate about the end of life when decisions were first made to remove organs from accident victims who were brain-dead but had healthy organs.

The potential benefits of stem cells should inspire optimism, but this must also be tempered with the frank admission that we still have far, far more to learn about embryonic stem cells. Unfortunately, the time required for the development of clinical treatments will be beyond that usually accepted by venture-capital investors, and it seems likely that a partnership will be needed between government sources of funds and private capital.

Anyone who knows or has cared for a person with an inherited or degenerative disease knows only too well the great need for new treatments. We should be excited by the opportunity rather than afraid.