 Dolly’s birth was a real breakthrough, for it proved that something that had been considered biologically impossible could indeed be done. Before Dolly, scientists thought that cell differentiation was irreversible: they believed that, once a cell has differentiated into a specialized body cell, such as a skin or liver cell, the process cannot be reversed. What Dolly demonstrated was that it is possible to take a differentiated cell, turn back its biological clock, and make the cell behave as though it was a recently fertilized egg.