The US Conference of Catholic Bishops objects to therapeutic cloning because it deliberately cuts off the life of a developing embryo for the purpose of extracting embryonic stem cells for research and, in theory at least, regenerative therapy: "In moral terms, we think that creating human lives solely to destroy them is more problematic than cloning for procreation" (Johnson 32).
But ethical concerns are not the only argument against cloning. Technical concerns also arise. Mammal cloning appears to have an "efficiency" rate of "approximately 2% of manipulated embryos, a number that seems not to be affected by the species used or the type of cell used as a nuclear donor." The pregnancies of cloned implants can be problematic, and the offspring may suffer from "a wide variety of abnormalities" (Iannaccone A469). In February 2003, for example, Dolly, cloned from adult sheep cells in 1996, was put down, suffering from diseases more typical of animals of more advanced age (Kolata A4). Meanwhile, in 2001, a group of researchers in Worcester, Mass., created what were said to be the first cloned human embryos for stem-cell research, but none of them developed beyond six cells (Johnson 34).
Some opponents of human reproductive cloning cite the "centrality of sexual recombination in mammal reproduction, and argue that it would be extremely difficult to predict either the vi