The world's first genetically modified monkeys have been created by scientists in the U.S by fusing cells from up to six different embryos.
These monkeys are the first time that chimeras - animals which are created in labs by made up of cells from two or more organisms - have been produced from a species other than rodents.
The rhesus monkeys, singleton Chimero and twins Roku and Hex, appear to be in good health with no apparent birth defects following the controversial technique.
The chimera monkeys, who were made up from a mixture of cells from as many as six individuals, could have be of "enormous" importance for medical research.
"The cells never fuse, but they stay together and work together to form tissues and organs," said co-author Shoukhrat Mitalipov of the Oregon Health and Science University in Portland, U.S.
While some rabbits and many "knock-off" mice have been made using embryonic stem cells cultured from a lab dish into a mouse embryo, this is the first the first time the same technique has worked for monkey embryos.
The technique could be a breakthrough for growing human organs from scratch and help to improve knowledge of IVF and stem cells.
The technique of "gluing" cells from very early stage monkey embryos before placing them into the womb of mothers has met by heavy criticism, who has describes the research as "deeply disturbing."
"Using such highly sentient animals in this research raises enormous ethical concerns and imposes a heavy welfare burden, resulting in severe suffering to many animals," says Dr Jarrod Bailey, scientific consultant for the British Union for the Abolition of Vivisection (BUAV).
"As few genetically modified animals show the 'desired' characteristics, many will be killed even before any research can take place, while others will die of severe and unrelated malformations caused by the genetic modifications.
"The monkeys who do exhibit characteristics of 'interest' are destined to suffer greatly by their very nature, and via the experiments to which they will be subjected."
The research has been reported in an online edition of the journal Cell.