In mice the success rate is slightly higher, about 3 percent. The main problem with cloning appears to be that in order to build a complete animal, the DNA of the donor cell has to be reprogrammed by the egg's environment so that all the genes can once again become active. That process appears to produce random mutations in genes. In some animals the mutations result in serious defects, thus shortly after birth many of them perish. Some cloned mice appear normal at first, but by the time they reach young adulthood, they exhibit extreme obesity.
Many cloned mice develop slower than normal.
Cloned cows have developed heart or lung defects. And even Dolly, the first cloned sheep, who initially appeared normal, began to show signs of obesity as compared with normal sheep, and had to be put on a special diet. In some animals the defects appear later in life. For instance, some cloned mice have appeared quite normal through young adulthood but then suddenly developed extreme obesity. Because of these serious health issues, while cloning could one day prove to be useful therapeutically, it is premature to use it on human beings.