Molecular Genetics of B Cell Development 
proximal variable-region segments, as well as the 
D and the J segments, are not affected. We are 
currently investigating the possibility that this 
positional effect is related to transcription and 
proximity to the immunoglobulin heavy-chain 
enhancer. 
,_In another area of investigation suggested by 
our initial experiments, we are trying to under- 
stand how a membrane-anchored immunoglobu- 
lin produces a signal. Although the structure of 
this receptor has been known for several years, 
the mechanism of signal transduction has not 
been elucidated. Recent work has implicated ty- 
rosine phosphorylation as well as GTP hydrolysis 
in the signaling event, and a series of immuno- 
globulin-associated proteins have been identi- 
fied. We have started to pursue this problem with 
the goal of developing a cell-free system that will 
allow us to isolate and reconstitute the functional 
components. 
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