"Serious concerns need addressing before attempting any clinical trial with iPS-derived cells; perhaps the most important is that of tumor formation," says Belmonte. This is because the virally delivered genes used to reprogram the skin cells can remain embedded in the cell's DNA even after reprogramming. These genes are thought to become active during the cell-differentiation process, considerably raising the long-term risk of cancer.

In recent weeks, however, scientists have published two new methods of making iPS cells that do not involve viruses and thus may overcome this problem.

Experts say that the research is an important proof of concept. "This is an exciting bit of science," says Chris Mason, a professor of regenerative medicine at University College London, who was not directly involved in the research. "It's likely to be the first of a slew of similar papers that may offer hope for conditions where today there is no real therapy, let alone a cure."

So far, Belmonte's approach is applicable only to diseases in which the genetic defect that underlies the disease has been identified. "But there are quite a few of these--and the number will increase," says Mason. Blood disorders are likely to be the first targets for therapy because corrected cells can easily be transferred back to the patient via bone-marrow transplants.

Belmonte adds that in the future, the correction of more-complex genetic disorders might become possible, thereby significantly increasing the number of diseases that might be treated with altered iPS cells.