Roku and Hex from OHSU News on Vimeo. Researchers have announced the birth of three unusual, though healthy, baby monkeys. They are the first non-mouse chimeras---creatures made up of cells from multiple other parents---to be created by science. Making chimeric mice is a time-consuming but fairly routine part of biology these days: embryos are injected with modified cultured stem cells containing the traits the researchers desire (like glowing in the dark). Those embryos grow up into mice who have some glow-in-the-dark cells and some normal cells, called chimeras. These chimeras are useful because if any of them have glow-in-the-dark sperm or eggs, they can be bred with each other to produce babies who are 100% glow-in-the-dark. These scientists tried to do the same thing with monkey embryos, but the cultured embryonic stem cells they injected didn't take. So instead, they jumbled together cells from 3 to 6 blastocysts---that's a very early stage of embryonic development, just after a fertilized egg---chose the 14 healthiest-looking resulting clusters, and implanted them into female monkeys. These three little guys---dubbed Roku, Hex, and Chimero---were the ones eventually delivered. They are all male, but some of their cells have female genomes. The team undertook this research because in mouse studies of the many different varieties of stem cells (embryonic, induced pluripotent, and so on) joining with embryos to form a chimera is the major test for whether a stem cell line really has the ability to form any kind of tissue. Their research seems to indicate that when it comes to primates, that test doesn't hold, meaning that primate stem cells may behave differently from mouse stem cells. And that's important, as mice, and not monkeys, are our primary model for understanding these possibly powerful tools for regenerative medicine. Moreover, the fact that making these little three guys required fresh embryonic cells, straight from the blastocyst, suggests that while research with such cells has been controversial, they may still be one of our best options for growing, for instance, replacement organs and tissues. Cultured stem cell lines or stem cells from adults have been advanced as an alternative, but if those won't work to make a chimera, that may indicate we'll have difficulty using them for other purposes.