From molecules to populations and back In biology, the most vigorous organisms often ensue from a union of two disparate, pure lines. In science, too, laws of hybrid vigor seem to operate at the interface between two disciplines, an interface that often proves to be fertile ground for germinating concepts and new outlooks. The fringes of research into the major histocompatibility complex (Mhc) have provided such an interface several times in the past and the encounters have invigorated fields such as transplantation biology, cellular immunology, and immunogenetics. In the last few years, a...