What’s the biological difference between an identical twin and a clone?

Identical twins have the same DNA as each other, but different from their parents. A clone, however, only has one parent and has exactly the same DNA as that parent. But even so, a clone isn’t a perfect copy. We now know that the way genes are turned on and off is greatly affected by the environment. Twins share the same uterus during development so they are exposed to the same mix of nutrients and hormones. A clone grows in a different uterus and at a different time from the parent animal, so it isn’t necessarily identical to the way its parent looked at the same age – even at birth.