Why did scientists once think adult mammal cells could not be used for cloning?

For decades, scientists presumed that adult mammal cells could not be used for cloning because of the nature of cell differentiation.1

As cells develop and specialize into specific types—such as skin or muscle cells—they lose their totipotency, which is the ability to become any of the different cell types required to form a complete, viable animal. Because this specialization involves permanent changes to gene expression patterns, researchers believed that the process of differentiation was irreversible. Consequently, it was thought that adult cells lacked the necessary genetic flexibility to guide the development of a new organism.12

This long-standing assumption was dispelled in 1996 with the birth of Dolly, the first mammal cloned from an adult cell. Her creation demonstrated that egg cells possess the remarkable ability to reprogram differentiated nuclei, effectively reversing the specialization process and restoring the potential for embryonic development.12