Molecular Genetics of the Major Histocompatibility Complex 
frequently than others. Placing this construct 
onto different genetic backgrounds may help to 
determine microrecombination frequencies in 
different mouse strains and perhaps identify criti- 
cal parameters in the process. These studies will 
contribute to our understanding of the genetic 
processes that control the evolution and ulti- 
mately the function of the mammalian immune 
system. 
Although sequence diversity and polymor- 
phism are the hallmark of genes, region genes are 
characterized by sequence conservation among 
alleles and limited polymorphism. The lack of 
polymorphism among these genes has been sug- 
gested to preclude an immunological function 
for the gene products. We have identified a re- 
gion gene w^hose sequence differs greatly be- 
tween alleles of the C57BL/6 and C3H mice. The 
sequence differences between the two alleles are 
manifested in both scattered and clustered nu- 
cleotide substitutions. The clustered substitu- 
tions are similar to those observed in the micro- 
recombinations that diversify genes and may 
reflect past microrecombination events with H-2 
and other Qa region genes. These data may pro- 
vide the first evidence that Qa genes can be recip- 
ients in the microrecombination process. 
Polymerase chain reaction (PGR) analysis has 
indicated that this Qa gene is transcribed in some 
strains of mice. We are currently engaged in an 
in-depth analysis of the transcription, translation, 
and cell surface expression of this gene and its 
product to ascertain its function. This gene is 
polymorphic in at least three strains of mice, and 
the analysis of other strains is under way. The di- 
versity and polymorphism of this gene suggest an 
immunological function for its product, perhaps 
the first Qa gene to be ascribed such a function. 
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