Rhesus monkeys Roku and Hex are two of the world's first lab-generated chimeric primates—each is composed of cells representing as many as six distinct genomes. Irresistible cuteness aside, the experiments that produced these monkeys are helping researchers better understand primate embryonic stem (ES) cells. For decades, scientists have used mouse ES cells to make chimeras. They add a few of these nondifferentiated cells into an early mouse embryo. The animal develops normally, incorporating the new cells into its tissues. When Shoukhrat Mitalipov and his colleagues at the Oregon National Primate Research Center in Beaverton tried the same test with rhesus ES cells, they found that the ES cells were unable to incorporate into the host embryo; the developing fetuses contained no trace of the added cells. The only way the researchers were able to make chimeric monkeys was to fuse several very early stage embryos into one.
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