The birth of Dolly1 has raised considerable interest and debate over the potential for cloning mammals, including humans. However, there are concerns about the authenticity of Dolly, and whether she could have been derived not from an adult donor mammary cell, but instead from a contaminating sheep cell culture or from a fetal cell present in the udder of the pregnant ewe donor2. Microsatellite typing suggested but did not prove authenticity1,2. We have therefore carried out a DNA fingerprint analysis to determine the origin of the donor cell used in nuclear transfer, and have confirmed the authenticity of Dolly.