Umbilical cord blood has a reputation for being the easiest source of adult stem stems. Businesses have even sprung up to store it at a child's birth as a kind of biological insurance policy. This year a British stem cell bank is touting its facilities as a Christmas present from doting grandparents. "Stem cells are not just for life; they're for Christmas," is the slogan of Smart Cells International.

However, the confidence of investors in this potential treatment has been shaken by the suspension of a clinical trial using cord blood by Viacell, an American biotech. Viacell was trying to use cord blood stem cells to treat blood cancers for patients who were compatible, but not perfectly matched. Two of the participants in the trial developed a severe adverse reaction.

Researchers are not sure where the problem lies. One possibility is the proprietary cocktail of growth factors used to multiply the stem cells. At the moment, scientists do not understand how to force stem cells to replicate themselves in a reliable fashion. Viacell's failure is not a setback for the field as a whole. A competing company, Gamida-Cell is due to begin a trial with its own therapy next year.