Molecular Genetics of Histocompatibility 
ity seems to be conferred by a complete tissue 
type (made up of the full complement of class I, 
II, and III genes) . In most cases the mechanism by 
which these inherited MHC genes affect disease 
susceptibility is not known. To understand the 
association of certain MHC alleles with disease 
susceptibility, it will be necessary to analyze the 
molecular structures of complete MHC com- 
plexes containing those alleles. Such a detailed 
analysis will also be required for an understand- 
ing of the mechanisms and forces that govern the 
evolution of the complex. Because this evolution 
determines our repertoire of immune responsive- 
ness, understanding these evolutionary forces 
will be important for understanding how we 
adapt to a world of constantly changing patho- 
gens, from simple viruses to complex parasites. 
Our laboratory is defining the structure of the 
entire MHC by molecular cloning. Large frag- 
ments of human chromosomes are propagated in 
yeast following fusion to simple yeast chromo- 
somal elements. These recombinant molecules 
exist in yeast as independent chromosomes (yeast 
artificial chromosomes [YACs]) . The use of these 
YACs permits us to analyze DNA fragments 5-10 
times larger than previously possible with con- 
ventional gene-cloning methods. This is particu- 
larly important for analysis of the structure of the 
MHC, because of the large size of the complex. 
Our analysis has shown YAC cloning to be a pow- 
erful methodology. All but 3 percent of the HLA 
complex has been isolated in 52 YACs. These mo- 
lecular clones are stable and reproduce faithfully 
the structure of the original human chromosome 
from which they derive. Their structure can be 
analyzed at high resolution. New DNA probes can 
be rapidly isolated from the YACs to be used for 
comparison of the structures of normal and dis- 
ease-associated MHCs. We anticipate that they 
will facilitate the identification of additional 
genes within the gene complex. Analysis of such 
genes may be critical for an understanding of the 
mechanisms of disease association. 
These studies provide access to the entire 
structure of the MHC. They also provide an exam- 
ple of the power of the new YAC-cloning technol- 
ogy that may facilitate its general application to 
the mapping of the human genome and to more 
extensive analysis of other specific human dis- 
ease loci. 
80 
