In August 2001 when President Bush forbade the creation of new embryonic stem cell lines with federal money, he softened the blow to biomedical research by promising that more than 60 ES cell preparations could still be used to develop prospective treatments for the sick. Yet a growing list of problems with those cells forces the Food and Drug Administration to consider whether material from them is even safe to try in people.

Only 22 of the sanctioned ES cell lines created before August 2001 have survived and remain available to researchers, although questions have arisen about their quality in light of their advancing age. The lines are supposed to be "immortal," but being kept in culture for extended periods has been known to induce deformities in other cells, so scientists were not entirely surprised when reports emerged of major genetic abnormalities in some of the National Institutes of Health registry lines. Other registry cell lines simply seemed to lose their ability to produce differentiated cell types or would only do so sluggishly.

Methods for handling stem cells have improved considerably since the US policy went into effect, and researchers believe that fresher cell lines can be kept much healthier. In particular, two new types of culture medium unveiled this year eliminate the need to grow ES cells on beds of mouse "feeder" cells, a practice used on all the government-approved lines in the past. Fears that the registry cells might have been contaminated with mouse molecules were recently confirmed by a study showing that human ES cells grown in this way absorb a mouse protein and carry it on their surface. When ES cells displaying the protein were exposed to human blood serum, antibodies against the animal protein attacked and killed the ES cells.

Nonetheless, California-based Geron, which owns rights to nine of the government-approved lines, says it will apply to the FDA early in 2006 for permission to go ahead with human trials of the cells for spinal repair. Thomas Okarma, Geron president, is confident the company's cells are clean after subjecting them to what he calls an "exhaustive list" of "gold standard" tests. No other US company has announced a formal application to try embryonic stem cell derivatives in people, but a director of the University of Minnesota's Stem Cell Institute, John Wagner, reportedly told state legislators last year his group had already sought FDA approval for such a trial. Wagner declined to reveal any more details.

Nor will the FDA comment on how many applications it has received for trials of ES cell derivatives or when it will rule. The possibility of animal contamination does not automatically preclude use of registry cells in humans--xenotransplantation of pig heart valves and even a baboon-to-human bone marrow transplant have gained FDA approval in the past. The only remark a spokesperson would offer was that the agency's decision will be based on the scientific soundness of the proposed trial, not politics.