Which cell was used to create Dolly the sheep?

Dolly the sheep was created using a mammary gland cell taken from an adult Finn Dorset ewe.1

To produce her, researchers at the Roslin Institute employed a technique known as somatic cell nuclear transfer (SCNT). In this process, the nucleus of the adult mammary cell was transferred into an unfertilized egg cell from which the original nucleus had been removed. The researchers used electrical pulses to fuse the mammary cell with the enucleated egg, triggering the resulting cell to begin dividing and developing into an embryo. This embryo was then implanted into a surrogate mother to complete gestation.1234