 Nuclear transfer can also be done using a donor cell from an embryo instead of from an organism after birth. Cloning mammals using embryonic cells has been successful since the mid-1980s (for a history of cloning, see Wilmut et al. 2001). Another technique to produce genetically identical offspring or clones is embryo twinning or embryo splitting, in which an early embryo is split in vitro so that both parts, when implanted in the uterus, can develop into individual organisms genetically identical to each other. This process occurs naturally with identical twins.