Back home in Israel at age 13, the boy's A-T was severe enough to require that he use a wheelchair when he also began complaining of headaches. Tests at Sheba Medical Center in Tel Aviv uncovered a growth pushing on his brain stem and a second on his spinal cord. Surgeons removed the spinal cord mass when the boy was 14, in 2006 and they say his general condition has remained stable since then.

But was the boy prone to tumors anyway or were the fetal stem cells to blame? A Tel Aviv University team extensively tested the tumor tissue and concluded it was the fetal cells. Among other evidence, some of the cells were female and had two normal copies of the gene that causes A-T—although that boy's underlying poor immune function could have allowed the growths to take hold.

Using stem cells from multiple fetuses that also were mixed with growth-spurring compounds "may have created a high-risk situation where abnormal growth of more than one cell occurred," wrote lead researcher Dr. Ninette Amariglio of Sheba Medical. She urged better research to "maximize the potential benefits of regenerative medicine while minimizing the risks."

This brain disease wasn't conducive to stem cell therapy in the first place, said stem cell specialist Dr. Marius Wernig of Stanford University, who said it's unclear exactly what was implanted.

"Stem cell transplantations have a humongous potential," Wernig said. But "if people rush out there without really knowing what they're doing ... that really backfires and can bring this whole field to a halt."