When we talk about human genetic engineering, cloning is only part of the issue – actually, cloning is a very small part of it, since at this point it's little more than an ethical debate. On the other hand, stem cell research has already produced tangible results. First thing's first – what are stem cells?
Simply put, stem cells are undifferentiated cells which can develop into specialized cells. Professor Sheldon Krimsky of Tufts University, Chairman of the Board of the Council for Responsible Genetics, explains why stem cell research is so promising:
There are certain cells in the body which cannot be replicated when they're damaged: like brain cells, heart cells, neurons. So if it's possible to have these primitive than that you can then [use to] repair damaged cells of the body, that you have a very new kind of medicine.
When we're talking about mammalian biology, there are two main kinds of stem cells – embryonic stem cells, which are found in embryos, and adult stem cells, which are found in tissues of adult organisms. Stem cell research is a young field, but it's been rapidly growing. At the beginning, mostly embryonic stem cells were used.
The main issue with embryonic stem cell research is the ethical one. Only fertilized eggs – or embryos – can be used to extract stem cells. Thus the process involves destruction of an embryo – even though the embryos are only days old, still some religions and groups believe it to be wrong, as these embryos would otherwise develop into human beings. In some places these beliefs transformed into legislative initiatives in some form or another. Some countries ban using embryonic stem cells, others allow with certain limits – for example, embryos cannot be older than several days. Interestingly, some countries do not allow destruction of embryos, but will allow work on stem cells extracted in other countries.
Stephen Hawking, who suffers from motor neurone disease, is a known advocate of stem cell research. In 2006 he said, criticizing political suppression of the research:
Stem cell research is the key to developing cures for degenerative conditions like Parkinson's and motor neurone disease from which I and many others suffer. The fact that the cells may come from embryos is not an objection because the embryos are going to die anyway. It is morally equivalent to taking a heart transplant from a victim of a car accident.
While these ethical questions remain a point for debate, perhaps they might be avoided in future. Adult stem cell research is a promising field, although currently there are some hurdles. For example, adult stem cells are restricted in what they can become. However, there is some evidence suggesting that there may exist adult stem cell plasticity, which would mean that this cell might have the potential to become a wider array of final cells. The greatest benefit of using adult stem cells is that it's not surrounded by controversy – in other words, while the process may be more complicated and applications at this time are limited, scientists do not have to jump through legal hoops or deal with ethical dilemmas on their road of discovery.