  In classical Greek and Egyptian myth, chimeras are depicted as having parts with the morphological characteristics typical of different species. The chimera of Greek mythology, for example, has the head of a lion, the body of a goat, and the tail of a serpent. Not all chimeras are mythical, however. In fact, each one of us is most likely a chimera, according to the modern scientific definition of a chimera as a single organism composed of cells with different embryonic origins (Nagy and Rossant 2001). As we interact with other organisms over the years, their cells become incorporated into our bodies, not only through a process of consumption and digestion, but also through processes that leave the cells intact. For example, pregnant women acquire cells from their fetuses and mosquitoes transfer blood between their successive victims.  More exotic natural chimeras occur when early-stage human embryos fuse in utero, resulting in a child with two distinct genotypes.  Laboratory-created chimeras include the geep, an intraspecies chimera made by fusing a sheep embryo with a goat embryo or by transplanting cells from one embryo into another (Fehilly, Willadsen, and Tucker 1984; Polzin et al. 1987). Like the chimeras of myth, the geep exhibits morphological traits of multiple species, with wool on some parts of its body and hair on others. Researchers have also created quail/chick chimeras by transferring parts of the neural tube (the precursor of the central nervous system) from a quail embryo into a Leghorn embryo, resulting in the transfer of vocalizations typical of a quail into the Leghorn chick (Couly, Coltey, and Le Douarin 1992). Intraspecies mice/mice chimeras, the most common of the laboratory-created chimeras, are typically created by genetically engineering embryonic stem cells derived from one mouse embryo and inserting them into another mouse embryo at the blastocyst stage. The modified embryos are then implanted into surrogate mice and brought to term. If the gametes (sperm or eggs) of the resulting chimeric mice are descendants of the genetically engineered cells, they can then be bred with each other to produce nonchimeric offspring carrying the genetic alteration in all of their cells.