US researchers said Thursday they have created the world\'s first genetically modified monkeys by fusing cells from up to six different embryos, in what could be a big advance for medical research. Until now, rodents have been the primary creatures used to make chimeras, a lab animal produced by combining two or more fertilized eggs or early embryos together. Scientists have long been able to create \"knock-out\" mice with certain genes deleted in order to study a host of ailments and remedies, including obesity, heart disease, anxiety, diabetes and Parkinson\'s disease. Attempts to do the same with more complicated primates have failed in the past, but scientists in the western state of Oregon succeeded by altering the method used to make mice. The breakthrough came when they mixed cells together from very early stage rhesus monkey embryos, in a state known as totipotent, when they are able to give rise to a whole animal as well as the placenta and other life-sustaining tissues. \"Knock-out\" mice are typically made by introducing embryonic stem cells that have been cultured in a lab dish into a mouse embryo, but that method failed in monkeys. Primate embryos do not allow cultured embryonic stem cells to become integrated, as mice do. Fusing primate cells apparently requires more potent, early stage cells from a living embryo, said lead researcher Shoukhrat Mitalipov of the Oregon National Primate Research Center at Oregon Health and Science University. The experiment produced three healthy male rhesus monkeys they named Roku, Hex and Chimero, with gene traits from all of the separate embryos used to meld them. \"The cells never fuse, but they stay together and work together to form tissues and organs,\" said Mitalipov. \"The possibilities for science are enormous.\" The research is published online ahead of the release of the January 20 issue of the journal Cell. Scientists use rhesus monkeys to study HIV/AIDS drugs, research vaccines for rabies, smallpox and polio, and to study potential uses for embryonic stem cells. They have also been launched into space on test missions by the US and Russia.