The modified polymer is an even more effective carrier. "We were able to give high levels of gene delivery even to hard-to-target cells," Anderson says. And the polymers deliver genes faster than viruses do.

Preliminary testing of the polymers in mice showed that they efficiently delivered a test gene coding for a fluorescent protein to ovarian tumors. "So far, everything we've done looks really safe," says Langer. The researchers are continuing to test the animals' immune responses to the polymer to ensure that it is safe. They are also testing in mice its safety and efficacy in delivering a therapeutic gene, one designed to kill cancer cells.

The ability to deliver genes only to specific cells and tissues is critical, says Anderson, especially when the gene is intended to kill the cells it enters, as with gene therapy for cancer. Langer and Anderson are confident that the polymer-gene complexes can be coupled with molecules such as antibodies to target specific types of cells in the body.

The MIT researchers have started by targeting cancer, but Langer says that in theory, there's "no limit" to what gene therapy can tackle. "I'm very excited," he says. If further animal tests continue to demonstrate the polymers' safety, Langer hopes to translate the work into gene therapies for patients.