Stem cells could be used to replace cells that are damaged or lost through disease or injury. A bone marrow transplant for a patient with leukaemia is one treatment already in use: the donor's stem cells replace the patient's faulty blood cells. Scientists hope that stem cells grown in a laboratory could one day treat diabetes, spinal cord injuries and some types of blindness. This is the main benefit.
There are also risks. Stem cells divide, so if they divide out of control they could form a tumour in the patient. Stem cells grown in the laboratory could carry a virus, which could be passed to the patient and cause an infection. If the donor's cells are not a close match, the patient's immune system may reject them. This rejection can make the treatment fail.
There are ethical issues too. Embryonic stem cells are taken from embryos, and the embryo is destroyed. Some people object to this because they think an embryo is a potential human life, while others support it because it could save or improve the lives of many patients.