The subject of cloning can instantly spark passionate debate: Will it enable us to resurrect beloved family members, or create Frankensteins? Rudolf Jaenisch wants to remove “hot air” from the discussion. His talk provides a clear picture of what is and is not scientifically feasible. Animal cloning, first pioneered in Dolly the sheep, used non-reproductive cells to create a carbon copy of the donor animal. This technique, tested many times, fails frequently and yields severe abnormalities. Consequently, Jaenisch believes the cloning of humans will never prove practical. But another variety of cloning generates far fewer genetic glitches and holds immense medical promise. The earliest cells of embryos can be manipulated to develop into neurons, or blood, or muscle, making them very useful tools in therapy for diseases like diabetes, Parkinson’s or leukemia. Jaenisch says these embryonic stem (ES) cells could be “an inexhaustible source of any tissue type and tailored to the needs of the patient.” But in spite of the potential rewards of this work, federal agencies and many others oppose it because so far the only source of ES cells has been human embryos. Jaenisch seeks a middle ground: scientists may not attempt cloning a human being, but may harvest and grow ES cells for therapeutic purposes from a human embryo.