The Major Histocompatibility Complex (MHC) was discovered as a con- sequence of the chronic problem encountered by cancer biologists in the early years of this century: the failure to maintain tumor lines by serial passage in outbred mice. A number of observations pointed to genetic similarity being a prerequisite for successful transplantation and they were incorporated into a genetic theory of transplantation by C.C. Little. This prompted scientists like Little to initiate breeding experiments designed to test his hypothesis and produce genetically identical mice which would permit the...