Stem cells are highly versatile cells in the human organism capable of both reproducing themselves and of developing to produce the more specialised cells out of which human tissue and organs are formed. At the current stage of scientific research, this offers the possibility of the injection of stem cells to repair damaged tissue, muscle, nerves or bone. Among the conditions which are potentially treatable are heart disease, Alzheimer's disease or strokes. For many years leukaemia has been successfully treated by the replacement of bone marrow, again an excellent source of stem cells.