Stem cells could be used to replace damaged cells and tissues. Stem cells from bone marrow are already used in transplants to treat blood diseases such as leukaemia. Scientists are also researching embryonic stem cells to treat conditions such as diabetes and paralysis caused by spinal injury, by growing new insulin-producing cells or nerve cells.
The benefits are that stem cells could treat diseases that are hard to cure at the moment and could replace damaged tissue. Embryonic stem cells can be taken from spare embryos left over from fertility treatment, which would otherwise be thrown away.
There are also risks. Stem cells grown in a laboratory can be contaminated with a virus, which can pass to the patient and cause a viral infection. Stem cells may divide out of control and form a tumour. If the cells are not genetically matched to the patient, the immune system may reject them.
There are ethical objections too. Taking embryonic stem cells destroys the embryo, and some people believe an embryo has the right to life. A decision about using stem cells means weighing the benefits against the risks and these ethical views.