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Chinese scientists have caused an uproar by trying to permanently edit the DNA of human embryos — created genetic changes that could be passed along from generation to generation.

Their attempt didn't work very well, but the report, published in a small, online journal called Protein & Cell, has worried experts who have been watching out for such experiments.

The motivation is to cure disease. In this experiment, the researchers were trying to correct defects in a gene called HBB that can cause a deadly blood disorder called beta-thalassemia.

Gene therapy in adults and children is still experimental; the idea is to fix faulty disease-causing genes. But done in a very early embryo, the repair, called germline editing, would be permanent. It could also be passed along to future generations.

That would be a good thing for curing disease, but if something went wrong, any mistakes would also be permanent. Gene therapy has been known to cause leukemia, for instance.

And it raises the specter of eugenics — making designer babies, or trying to "improve" the human race.

"Human...