Semi-identical twins discovered
Scientists have revealed details of the world's only known case of "semi-identical" twins.
The journal Nature says the twins are identical on their mother's side, but share only half their genes on their father's side.
They are the result of two sperm cells fertilising a single egg, which then divided to form two embryos - and each sperm contributed genes to each child.
Each stage is unlikely, and scientists believe the twins are probably unique.
These twins were born in the US, but neither their identity or their exact location is being revealed.