By Paul Basken

Federally sponsored research conducted with human embryonic stem cells has largely involved only two genetically distinct lines, a Stanford University researcher has found. This raises pressure on the Obama administration to approve their use, along with concerns that stem-cell research has a diversity problem.

The "startling near monopoly of just two cell lines" suggests a threat to research work nationwide if those lines cannot be shown to meet the new federal standards, Christopher T. Scott, director of Stanford's program on stem cells in society, reported on Friday in the journal Nature Biotechnology.

Bush-administration restrictions over the past eight years made only 21 stem-cell lines eligible for research in laboratories receiving federal financing. But of the 1,217 requests for lines from the National Stem Cell Bank from March 1999 to December 2008, a total of 941, or 77 percent, involved just two lines, Mr. Scott reported. A third line involved 111 requests, and no other exceeded 35, he found.

President Obama announced in March that he would end the Bush-administration restrictions, though he added his own conditions on federal support for embryonic-stem-cell research, including a requirement that the experimentation was explained to the couples who produced the embryos.

It is not clear whether the two most popular lines identified by Mr. Scott will meet Mr. Obama's new standards. "If none are ultimately approved, it would be at a very considerable cost," said James A. Thomson, the University of Wisconsin biologist known for deriving the first human embryonic-stem-cell line.

"But the field would nonetheless survive and move on as new cell lines are derived in accordance with the new standards," Dr. Thomson said.

Embryonic stem cells, because of their potential to grow into any of more than 200 types of tissue in the body, raise the possibility of cures for a range of ailments that include cancer, diabetes, and heart disease. They are typically obtained from the excess embryos created by couples who have been trying to start a pregnancy with the help of a doctor.

The findings by Mr. Scott may also have implications for the quality of embryonic-stem-cell research. The reliance by scientists on just two lines raises the possibility that findings to date may lack reliability because they involve such a limited genetic pool, he said.

The representative nature of the 21 eligible lines may already have been skewed by political and social factors involving the couples likely to have created embryos, Mr. Scott said. On the other hand, he wrote in his report, the "reproducible yet small number of well-characterized lines are now used as references for the community of stem-cell researchers."

The diversity concern had long troubled scientists opposed to the Bush-administration restrictions, said Arnold R. Kriegstein, director of the Eli & Edythe Broad Center of Regeneration Medicine and Stem Cell Research at the University of California at San Francisco.

Mr. Scott's findings may also be cause for hope, raising the odds that medical solutions may now be found in a more diverse set of embryonic stem cells, and through better comparisons with other methods of generating stem cells that don't involve embryos, Dr. Kriegstein said.

"There are so many unanswered questions that we just really don't know," he said. "Over time it will become clear what the diversity represents."