Proposed stem cell experiments would involve creating a human-mouse hybrid to test different lines of human embryonic stem cells for their quality and potential usefulness in treating specific diseases.

Any animals born from the experiment would be chimeras — organisms that are mixtures of two kinds of cells, such as a mouse with a brain made entirely of human cells or a mouse that generated human sperm. However, Dr. Irving L. Weissman of Stanford said these undesirable outcomes could be avoided by deleting certain genes from the human cells before injecting them into a mouse.