A sheep embryo can be engineered to produce a certain human protein or hormone in its milk. The human protein can then be harvested from the milk and sold on the market. Instead of trusting the somewhat unpredictable and time-consuming methods of normal animal husbandry to reproduce this genetic hybrid, cloning it assures that the engineered gene product will not be lost. Based on attempts in frogs and mice, most scientists felt that the reprogramming was impossible. A critical question is the lifespan of Dolly.
All cells have a built-in senescence or death after so many cell divisions.
Dolly began from a cell that was already six years old. A normal lifespan for an ewe is around 11 years. It is also uncertain as to whether Dolly will be reproductively fertile. Frog clones are usually sterile. Reprogramming the nucleus could lead to procedures to stimulate degenerating nerve cells to be replaced by newly growing nerve cells.