Several readers tipped news that U.S. scientists have created 'chimeric' monkeys, made with genetic material from as many as six different genomes (abstract). This is significant because it's the first time researchers have used the technique on a primate. From the article: "Researchers took very early stem cells, called totipotent stem cells, from separate developing embryos and basically glued them together, implanting the mixed embryos into surrogate mother monkeys. The cells — from totally different sources — didn’t fuse, but worked together in harmony, forming fully fledged, normal, healthy animals. ... The key here was the scientists’ use of totipotent cells, so named for their ability to differentiate into the totality of possible cells in an animal. A totipotent cell can give rise to a whole animal. Pluripotent stem cells, the type most frequently used in stem cell research, can differentiate into any cell in the body, but can’t become a whole animal, and can’t make other embryonic tissues like a placenta. Totipotent stem cells are only derived from the very earliest stages of a zygote, mere days after fertilization. In humans, totipotent cells differentiate into pluripotent cells after four days."