Dealing with DMA on a Large Scale 
from yeast into human cells. We have concen- 
trated on YACs containing two human genes, 
HPRT and GART. Both code for enzymes required 
for the synthesis of nucleic acid precursors in hu- 
man cells. Rodent cell lines are available that do 
not produce these enzymes. We have succeeded 
in transferring YACs containing the human HPRT 
and GART genes into these rodent cells and have 
shown that the genes are expressed. A variety of 
gene transfer methods have been used, including 
microinjection and direct fusion of yeast sphero- 
plasts (cells from which the walls have been re- 
moved) with rodent cells. 
Physical analysis of DNA cloned into YACs also 
remains a major research interest. YACs have 
greatly improved our ability to recover DNA from 
higher organisms in megabase-pair blocks. How- 
ever, much of the biological interest in these 
blocks occurs at the level of the base pair, where 
a single change can have profound conse- 
quences. The problem of analyzing megabase- 
pair blocks of DNA at base-pair resolution re- 
mains formidable, particularly since there is a 
need for generic solutions that can be applied to 
any large segment of DNA from any organism. Fur- 
thermore, given the amount of DNA present in 
the genome of higher organisms and the central 
role that its analysis plays in biomedical research, 
it is essential that these techniques rely primarily 
on advanced instrumentation rather than skilled 
laboratory personnel. The challenges and oppor- 
tunities in this area are reminiscent of those faced 
by digital computing in the 1950s: promising 
technology was already in hand, but vastly im- 
proved efficiency was required before its prom- 
ise could be broadly realized. 
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