The Regulation of Development 
controlled gene dosage, and the result was 
lethality. 
Our laboratory is also developing techniques 
to study the large number of mutations affecting 
development that have been described in the 
mouse. Few of these have been approached at the 
molecular level, primarily because there were no 
experimental tools available to gain access to the 
mutated genes. The advent of techniques that al- 
low the investigator to map and clone megabase 
amounts of DNA at a time — specifically pulsed- 
fiield gel electrophoresis and yeast artificial chro- 
mosomes — has eliminated this constraint. We 
have begun a long-term project to develop tech- 
niques that will allow us to clone such genes and 
to establish their identities by complementation 
analysis via introduction of yeast artificial chro- 
mosomes into embryonic stem cells. 
A complete library of mouse DNA in yeast, 
currently under construction, is being used to 
isolate the mouse Fused gene, which affects 
neural ectoderm development in the early mouse 
embryo. A 650-kb segment of DNA has been 
cloned near the FusedXocus, and a large interspe- 
cies backcross is being used to localize its rela- 
tionship to the Fused gene itself. To prove that 
the Fused gene has been identified, large seg- 
ments of DNA containing it will need to be intro- 
duced back into the mouse germline. Strategies 
for accomplishing this through embryonic stem 
cells are in progress. 
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