When George W. Bush three years ago announced that federal funding for stem-cell research would be restricted to 64 existing stem-cell lines, it sounded like a reasonable number, at least to accomplish preliminary work. Scientists, however, have determined that many of those lines are not viable, and now two new studies have found that the last couple of dozen have their own problems. Given the promise of medical breakthroughs, it is time for the federal government to lift the funding restrictions.

The first of the new studies, which is awaiting publication, reports that the remaining stem-cell lines have a problem that dates back to their creation. In the early days of stem-cell research, human cells were cultivated piggybacking on mouse cells. There has always been concern that the proximity to mouse cells might have corrupted the lines, making them unsuitable for human use. The original concern was that a mouse-specific virus might have infected the human cells, but the new study finds another mouse problem. It turns out that certain mouse molecules have lodged on the surface of the human cells. If transplanted into a person, that person’s body would see those cells as invaders and attempt to destroy them.

The second study further complicates the situation, finding that the existing cell lines each have a propensity to develop into one kind of cell or another. That means that the possible applications for the lines are extremely restricted.

And broad application of stem cells is one of the great promises of the field. Researchers hope that treatments for disparate physical maladies will come from stem cells. For example, they might help sufferers from Alzheimer’s, diabetes and crippling nerve damage. Practical uses are probably still years away, but they will not happen unless researchers are unhindered by federal restrictions.

To be sure, serious stem-cell research is taking place, just not in the United States. Countries such as South Korea, the United Kingdom and Australia are leading the drive in what could be the greatest medical breakthrough in decades. If the restrictions remain, Americans will face needless delays when treatments are ready for human use, assuming that federal officials even allow doctors to use the treatments here.

One of America’s great strengths is its commitment to research and innovation. Many of the most important discoveries of the last 200 years have their roots on our shores. That success relies in no small part on financial backing from the federal government, money that pays for expensive research that will benefit many people. The federal government should stop hamstringing work on new stem cells lines that hold great promise for sick Americans.