Recombinant DNA was the first in a series of ever-improving steps for manipulating genetic material. The chief problem has always been one of accuracy, of editing the DNA at precisely the intended site, since any off-target change could be lethal. Two recent methods, known as zinc fingers and TAL effectors, came close to the goal of accurate genome editing, but both are hard to use. The new genome-editing approach was invented by Jennifer A. Doudna of the University of California, Berkeley, and Emmanuelle Charpentier of Umea University in Sweden.

Their method, known by the acronym Crispr-Cas9, co-opts the natural immune system with which bacteria remember the DNA of the viruses that attack them so they are ready the next time those same invaders appear. Researchers can simply prime the defense system with a guide sequence of their choice and it will then destroy the matching DNA sequence in any genome presented to it. Dr. Doudna is the lead author of the Science article calling for control of the technique and organized the meeting at which the statement was developed.

Though highly efficient, the technique occasionally cuts the genome at unintended sites. The issue of how much mistargeting could be tolerated in a clinical setting is one that Dr. Doudna’s group wants to see thoroughly explored before any human genome is edited.

Scientists also say that replacing a defective gene with a normal one may seem entirely harmless but perhaps would not be.

“We worry about people making changes without the knowledge of what those changes mean in terms of the overall genome,” Dr. Baltimore said. “I personally think we are just not smart enough — and won’t be for a very long time — to feel comfortable about the consequences of changing heredity, even in a single individual.”

Many ethicists have accepted the idea of gene therapy, changes that die with the patient, but draw a clear line at altering the germline, since these will extend to future generations. The British Parliament in February approved the transfer of mitochondria, small DNA-containing organelles, to human eggs whose own mitochondria are defective. But that technique is less far-reaching because no genes are edited.

There are two broad schools of thought on modifying the human germline, said R. Alta Charo, a bioethicist at the University of Wisconsin and a member of the Doudna group. One is pragmatic and seeks to balance benefit and risk. The other “sets up inherent limits on how much humankind should alter nature,” she said. Some Christian doctrines oppose the idea of playing God, whereas in Judaism and Islam there is the notion “that humankind is supposed to improve the world.” She described herself as more of a pragmatist, saying, “I would try to regulate such things rather than shut a new technology down at its beginning.”