Whether scientists can capitalise on the huge potential that stem cell research and therapeutic cloning promise depends on where in the world they work. There is a disparate and confusing patchwork of legislation, with little agreement between countries on exactly what should be permitted and what should be banned. Attempts to reach consensus have failed in Europe and at the United Nations, and in some countries the debate remains unresolved at the national level.

The science is complex, and the ethical dimensions equally so. But the problem lies in the major differences of opinion over which parts of the science are considered acceptable.

There are three main scientific issues at the heart of the debate--human embryonic stem cells, reproductive cloning and therapeutic cloning. To some, all three are equally unacceptable, but to others they are different enough to merit separate consideration.

The source of human embryonic stem cells is a major point of contention, as they are taken from embryos that are just a few days old. They are primarily taken from embryos that have been left over from fertility treatments, but this limits the types of research that can be carried out. A possible alternative, and one that raises further moral quandaries, is to produce cloned embryos.

Since the cloning of Dolly the sheep in 1997, the world has had to grapple with the serious prospect that cloning a human might indeed be possible. The single point which all countries seem agreed upon is that, for now, attempting to create a human clone, also known as reproductive cloning, is scientifically unsafe, ethically unsound and unacceptable socially.

But there is a related procedure known as therapeutic cloning whereby the early embryo never develops beyond a microscopic ball of cells in the laboratory. During this time, research is carried out on it, most often to extract stem cells, but it can also be to understand better the early development of genetically based diseases.

Some countries have put in place total bans on all forms of human cloning, others have banned reproductive cloning but still allow therapeutic cloning and some have so far failed to introduce any regulations, often as the result of a failure to reach agreement. Many countries also have regulations on the derivation and use in research of human embryonic stem cells.

To illustrate the range of regulation, we can look at the huge differences between the US and the UK.

The UK is one of a handful of countries to have introduced legislation with the express purpose of allowing the use of human embryos for stem cell research and therapeutic cloning. In 2001 the UK introduced primary legislation against reproductive cloning; however, this action was taken after it had extended the terms of the Human Fertility and Embryology Act governing licensed research on early human embryos.

These measures were taken following wide public debate and were passed by majorities of more than two to one in both Houses of Parliament. The Royal Society, as the UK's national academy of science, played a significant role in informing the debate during this process. The result has been a carefully regulated process, which has so far resulted in two licences being granted to carry out research into diabetes and into motor neurone disease.

By stark contrast, in the US, despite an influential religious lobby consistently condemning any research involving embryos, there is no primary federal legislation to regulate any form of human cloning. This reflects a split between those who strongly believe all cloning should be banned and those who wish to see only reproductive cloning banned and an inability to come up with suitable legislation, despite numerous and ongoing efforts.

The latest development was the re-submission of the Human Cloning Prohibition Act of 2005 to Congress by Senator Sam Brownback of Kansas on March 17. This proposed a federal ban, which makes no distinction between reproductive and therapeutic cloning and has strong support but has already failed to make it into law twice since 2001. Brownback has also declared his equally strong opposition to any effort in the House of Representatives to reconsider an existing ban on federal funding of some embryonic stem cell research.

Worryingly, no federal legislation exists to stop a privately funded laboratory attempting to create a human clone. But any outcome of research would then be subject to Food and Drug Administration approval, which it would be extremely unlikely to pass.

Scientists can receive federal funds to use human embryonic stem cells in their research, but only the cell lines created prior to 2001, of which only 22 are available. Also, some states have now enacted their own legislation, in some cases to ban all cloning and embryonic stem cell research and in others to allow therapeutic cloning and even pledge millions of dollars of funding, most notably in California.

Countries where therapeutic cloning and stem cell research are permitted often regard it as great news that the US is lagging behind. Levels of investment in this kind of research in the UK are testament to this. But in the long term, losing out on the expertise and resources of the world's leading scientific nation means patients around the world will lose out, too, because a global effort is needed to make the most rapid progress.

Elsewhere, the opinions and legislation are equally varied. Europe is divided on the issues. Most countries, including Germany, Austria, France and the Netherlands, have brought in legislation to ban reproductive and therapeutic cloning. Yet they are in the curious position of not going as far as countries such as Italy, Ireland, Norway and Denmark, which have also restricted research using human embryonic stem cells. This raises an interesting moral question of whether these nations will allow their patients to receive the treatments developed in the future using technologies that they consider unacceptable.

Belgium, Sweden and Spain allow therapeutic cloning and human embryonic stem cell use in similar frameworks to the UK, and there is now public pressure in Germany and Italy to revisit their legislation, while Ireland is already doing so.

In Asia, the picture is very different. Japan, China, Singapore and South Korea all follow the UK's approach. India is embracing human embryonic stem cell research, as realised recently at an Indo-UK meeting organised by the Royal Society and aimed at spawning international collaborations in the field. But so far it still has a ban on therapeutic and reproductive cloning.

South America is as divided as Europe. Ecuador bans embryonic stem cell research and both types of cloning; Brazil bans cloning, but a new law allows and funds embryonic stem cell research; Argentina, Chile, Peru and Uruguay ban both types of cloning, and legislation either allows or does not cover embryonic stem cells, and only Colombia permits therapeutic cloning as well as human embryonic stem cell research.

In the Middle East, only Israel and Turkey have any relevant legislation. Israel permits therapeutic cloning and embryonic stem cell research while banning reproductive cloning. Turkey has effectively the same--although stem cell research is not explicitly permitted, it is just not mentioned.

On the continent of Africa, only South Africa (embryonic stem cell research--yes; both types of cloning--no) and Tunisia (embryonic not specifically prohibited; both types of cloning--banned) have enacted laws.

For the countries that do not have national legislation we can gain an idea of their attitudes from the ill-fated attempts to gain consensus at the European and international levels.

The Council of Europe has introduced the ambiguous European Convention on Human Rights and Biomedicine. It is not clear whether it bans therapeutic cloning. Thirty-one of the 45 member states have signed, of which 15 have also ratified. In response to the debate in the UK, which preceded the introduction of its legislation on cloning, an additional Protocol on the Prohibition of Cloning Human Beings was drafted to try to influence the outcome. Unsurprisingly, the UK has not signed either, but as neither the convention nor the protocol gives any sanctions for violation it is unlikely to have any major effect. Portugal, though, has signed and ratified the convention, despite no national legislation, which is a likely indication of its views.

At the United Nations we see a similarly confused picture. A committee was formed in 2001 to consider "the elaboration of an international convention against the reproductive cloning of human beings". Four years of stop-start debate and negotiations saw member states unable to get anywhere near a consensus on whether therapeutic cloning should be included in the ban.

One of the most influential groups during the tail end of the debate was the Organisation of Islamic Countries (OIC). It is suspected that part of the reason that those seeking a ban on all forms of cloning, such as the US and Costa Rica, did not push for a convention was because of a last-minute indication that the OIC would support an alternative proposal. Initiated by Belgium and supported by the UK, the proposal asked that individual countries be allowed to make their own decision on therapeutic cloning.

Instead the result was a poorly worded and ambiguous political declaration that appears to ban all forms of cloning. But because it is nonbinding, it will have absolutely no effect on countries that wish to forge ahead with therapeutic cloning.

Unfortunately, this outcome also means that no clear message has been sent to maverick scientists that the entire world believes that reproductive cloning is unacceptable.