The mythological Chimera is described as having the head of a lion, the body of a goat, and the tail of a serpent. In medicine, the term “chimera” is used to indicate a body that contains cell populations derived from different individuals. To pioneers in the development of tests of maternal peripheral blood for prenatal diagnosis, persistent low levels of chimerism (microchimerism) with fetal cells from an earlier pregnancy was an impediment to accurate diagnosis. This impediment was cast in a different light by studies of peripheral blood from normal women who had previously given birth; male DNA was identified . . .