Clones' clan. Two cloned mice stand by their white surrogate mother (lower tier), with the egg and nucleus donors above (top right and left, respectively).

Cloned Mice Keep Dolly Company

As the first mammal cloned from an adult cell, Dolly was greeted first with awe and, later, with doubts, for some researchers wanted more proof that she was the real McCoy. Now they have it. A report in tomorrow's issue of Nature confirms for the first time that cloning from adult cells is not only possible but repeatable. These new experiments have so far yielded more than 50 cloned mice.

As with Dolly, the mice were cloned by putting nuclei from adult cells into eggs whose own DNA had been removed. The resulting cells were then triggered to develop into embryos, which were implanted in foster mothers. Dolly was produced by fusing the donor and egg cells and activating development with an electrical pulse, while Ryuzo Yanagimachi and his team at the University of Hawaii, Honolulu, injected nuclei into mouse eggs using a very fine needle. They then let the cells sit for up to 6 hours before triggering the eggs to develop by putting them into a culture medium containing strontium, which stimulates the release of calcium from the eggs' internal stores--the same signal that tells fertilized eggs it's time to start dividing.

Yanagimachi confirmed that the newborn mice had been cloned by comparing their DNA to that of the animals that provided the nuclei. It was identical. And the cloned mice seem normal--as were clones of clones and offspring of clones mated to each other. All told, "it's a very compelling paper," says Michael McClure, a cell biologist at the National Institute of Child Health and Human Development in Bethesda, Maryland.

Meanwhile, two other reports in the same issue describe DNA analyses proving that Dolly and the ewe she was cloned from are indeed genetically identical, as would be expected of clones. And in Japan, two calf clones born 5 July but not yet fully described in the scientific literature have apparently passed similar tests. Cloning is "a real phenomenon," comments Richard Schultz, a developmental biologist at the University of Pennsylvania in Philadelphia.