The National Academy of Sciences has proposed ethical guidelines for research with human embryonic stem cells.

The report paves the way for research involving animals called chimeras that have been seeded with human cells. The purpose of such experiments is to test first in animals the human organs that could be grown from embryonic stem cells.

But it places certain types of experiments out of bounds, at least for now. These involve inserting human embryonic stem cells into an early human embryo, a technically promising method of genetic engineering, or into apes and monkeys.