Detection of Genetic Recombination in Germ Cells 
recombinations more frequently than others. Our 
goal is to determine microrecombination fre- 
quencies in several mouse strains and perhaps 
identify critical parameters in the microrecom- 
bination process. These studies will contribute to 
our understanding of the genetic processes that 
control the evolution and ultimately the function 
of the mammalian immune system. 
Although sequence diversity and polymor- 
phism are the hallmark of H-2 genes, Qa and Tla 
region genes are characterized by sequence con- 
servation among alleles and limited polymor- 
phism. The lack of polymorphism has been sug- 
gested to preclude an immunological function 
for the products of these genes. We have identi- 
fied a Qa region gene whose sequence differs 
greatly between alleles of the C57BL/6 and C3H 
mice. The sequence differences between the two 
alleles is manifested in both scattered and clus- 
tered nucleotide substitutions. The clustered 
substitutions, which are similar to those observed 
in the microrecombinations that diversify H-2 
genes, may reflect past microrecombination 
events with /f-2 and other Qa region genes. These 
data may provide the first evidence that Qa genes 
can be recipients in the microrecombination 
process. 
PGR analysis has indicated that this Qa gene is 
transcribed in some strains of mice. We are en- 
gaged in an in-depth analysis of the transcription, 
translation, and cell surface expression of this 
gene and its product in order to ascertain its func- 
tion. This gene is polymorphic in at least three 
strains of mice, and other strains are being ana- 
lyzed. The diversity 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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