One is a singleton named Chimero and there are twins named Roku and Hex, after the word "six" in Japanese and Greek. All three monkeys are biologically male, but blood tests show that Roku has both male and female cells.
The cells that make up their tissues and organs are made of cells from each of the contributing embryos. "The cells never fuse, but they stay together and work together to form tissues and organs," said Shoukhrat Mitalipov, the lead researcher.
Mitalipov and his team created the monkeys by putting four-day-old embryos together in a culture dish, then letting them grow for a few days into early-stage embryos called blastocysts. As the Guardian explains,
The researchers implanted the chimeric embryos into five female rhesus monkeys, all of which became pregnant. Tests on the foetuses confirmed that all of the animals' organs and tissues contained cells from more than one embryo.
The Guardian writes that the first chimeric animals were created in the 1960s -- first with mouse embryos, and later with rats, rabbits, sheep and cattle. (There are also naturally occurring human chimeras.)
"Stem cell therapies hold great promise for replacing damaged nerve cells in those who have been paralyzed due to a spinal cord injury or for example, in replacing dopamine-producing cells in Parkinson's patients who lose these brain cells resulting in disease," Mitalipov said in a press release.
Watch this video showing how the cells were manipulated: