Dolly, a female domestic sheep, was the first successful clone derived from an adult mammal cell. dolly showed that genes in the nucleus of a mature differentiated somatic cell are still capable of reverting to an embryonic state

Dolly was a first mammal cloned from an adult somatic cell, she was a domestic sheep, using the process of nuclear transfer. Dolly showed that genes in the nucleus of such a mature differentiated somatic cell are still capable of reverting to an embryonic totipotent state.
The totipotent cell is a cell that can form all the cell types in the body, it has an ability of a single cell to produce and divide all of the differentiated cells in an organism.