| In Greek mythology, a chimera is a part-lion, part-goat, part-snake, all-weird monstrosity, but in scientific circles, the name takes on a different meaning. In biology, a chimera is anyone whose body contains genetically distinct cells. This doesn't mean that another small organism is living within you, but rather that cells wholly different from your cells coexist within you. In fact, a kind of chimerism is fairly common. New mothers carry some of the cells of their offspring, which can remain within their body for up to 40 years. This exchange of cells is also a two-way street, as children often contain some of their mother's cells, which cross the placenta and into the child's bloodstream during pregnancy. However, not many cells reside in a mother and her offspring, so scientists refer to this phenomenon as "microchimerism." A more dramatic form of human chimerism is when two embryos that would usually form nonidentical twins instead fuse in the womb, and the resulting single embryo contains cells of both. While this can have some outward effects (like different-colored patches of skin on one person), most of the time this condition is only discovered through genetic testing. In one bizarre case in 2003, a 52-year-old mother needed a kidney transplant, and when her children were tested for compatibility, the results showed that two of her three children were not hers genetically. This is because she was a chimera, having fused with her twin embryo before birth, and the doctors didn't think to search for a second genetic marker in other parts of her body. That makes this confused mother only 1 out of 100 or so confirmed chimera cases worldwide, but many, many more are likely out there. | 
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