Most researchers had thought mice would be particularly difficult to clone, because the DNA in mice embryos switches on very early, possibly as soon as the egg splits into the two-cell stage.

• 大多数研究人员曾认为克隆老鼠非常困难，因为老鼠胚胎的DNA转换很早，可能在卵子分裂为两个细胞的阶段就进行了。