Feng Zhang of the Broad Institute of MIT participates in a panel discussion at the National Academy of Sciences international summit on the safety and ethics of human gene editing, Tuesday, Dec. 1, 2015, in Washington. Alternating the promise of cures for intractable diseases with anxiety about designer babies and eugenics, hundreds of scientists and ethicists from around the world began debating the boundaries of a revolutionary technology to edit the human genetic code. (AP Photo/Susan Walsh)

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