NEW YORK • The National Institutes of Health has announced that it is planning to lift its ban on funding some research that injects human stem cells into animal embryos.

The NIH announced on Thursday its proposal in a blog post by Ms Carrie Wolinetz, associate director for science policy, and in the Federal Register.

The purpose is to try to grow human tissues or organs in animals to better understand human diseases and develop therapies to treat them.

Researchers have long been putting human cells into animals - like pieces of human tumours in mice to test drugs that might destroy the tumours - but stem cell research is fundamentally different. The stem cells are put into developing embryos where they can become any cell, like those in organs, blood and bone.

If the funding ban is lifted, it could help patients by, for example, encouraging research in which a pig grows a human kidney for a transplant.

But the very idea of a human-animal mix can be chilling, and will not meet universal acceptance.

In particular, when human cells injected into an animal embryo develop in part of that animal's brain, difficult questions arise, said Mr Paul Knoepfler, a stem cell researcher at the University of California, Davis.

"There's no clear dividing line because we lack an understanding of at what point humanisation of an animal brain could lead to more human-like thought or consciousness," he said.

The NIH's plan will most likely go into effect in autumn - perhaps with some modifications - after a 30-day comment period that is now open to the public and researchers.

The NIH, which would be a major source of federal funds for this type of work, imposed the moratorium last September to consider concerns about the research.

But Ms Renate Myles, a spokesman, said, "We watch the state of the science and knew that this was where the science was heading."

Two months later, the NIH convened a workshop to hear from researchers and experts in animal welfare. The NIH would keep its ban on funding any research that could result in an animal with human sperm or eggs that would then be bred.

Mr Jeffrey P. Kahn, director of the Johns Hopkins Berman Institute of Bioethics, points to two looming ethical issues.

One is to decide if there is a fundamental difference between adding DNA from one species into another - the technology used to produce genetically modified foods - and putting human cells into an animal.

Where to draw the human boundary is another issue. "What are we doing when we are mixing the traits of two species?" Mr Kahn asked.

"What makes us human? Is it having 51 per cent human cells?" Those questions, he added, "are part of what make people react to this issue".

NEW YORK TIMES