This image released by the Oregon Health & Science University (OHSU), shows new-born, genetically-modified monkeys Roku (Left) and Hex. US researchers said that they have created the world's first genetically modified monkeys by fusing cells from up to six different embryos, in what could be a major advance in 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.