Dr Simon Best of biotechnology company Geron Biomed and Dr Patrick Dixon, author, discuss human cloning

The work could lead to new treatments for diseases and illnesses ranging from leukaemia, stroke Parkinson's and Alzheimer's disease and heart attacks. Two scientific panels advised the government last year that therapeutic cloning could be of great benefit to seriously ill people.

The technology centres on human embryonic stem cells. These are the "master" cells which have the potential to become any of the body's many different types of tissue.

Cloning is the only way to get stem cells say some scientists

Scientists believe that if they can grow these cells in the laboratory and then control the way they develop, they could, theoretically, grow any type of tissue needed for transplant. The great advantage of this approach would be that the transplanted tissue would be a perfect-match for the patient and would therefore not be rejected.

For example, a healthy skin cell could be taken from a leukaemia sufferer and cloned by creating a human embryo. The stem cells produced by the embryo could then be grown and multiplied into bone marrow cells for transplantation.

Cloning essential

But this treatment cannot happen without cloning, some scientists argue. Only embryos producing the early-stage cells which have the potential to develop into any tissue cell.

Scientists in America have already succeeded in isolating and growing cultures of human embryonic stem cells. Details of human embryos cloned in November 1998 by Advanced Cell Technology were released recently.

The BBC's Alistair Jackson: "Special cells have the potential to produce limitless skin, nerves and muscle"

However, other recent research has shown that some later-stage stem cells are present in human adults. Opponents of cloning argue that these could be used to create the perfect-match transplant tissue and that cloning is not only ethically wrong but also unnecessary.

Cells not babies

The type of human cloning suggested by the two scientific panels to aid medical research is called "therapeutic" cloning. It is likely that the age limit for these human embryo clones will be no more that 14 days. At that point they would be destroyed.

Therapeutic cloning is expressly not aimed at creating living copies of human beings. This "reproductive" cloning should be specifically banned by law said the two scientific panels.

How genetically old is Dolly?

The scientific challenges involved in bringing a healthy human clone through a pregnancy to birth are huge. Current cloning success in large mammals is at best one in every hundred or more attempts.

The laboratory at Roslin, Edinburgh which cloned the sheep Dolly have so far been unable to produce a pig clone. There is also doubt over the longer-term health of adult clones, which could suffer from genetic defects and premature ageing.

However, cloning opponents believe that any change in the law which allows therapeutic cloning will allow scientists the refine their methods and make the likelihood of the birth of a human clone much greater.