Regulation of Gene Activity During B Cell Development 
molecule (antigen receptor) and are required for 
the expression of the receptor on the cell surface. 
The mb-l gene is activated early in B cell ontog- 
eny, continues to be expressed in mature B cells, 
but is turned olF in plasma cells. We have identi- 
fied the mb-l promoter and shown that it con- 
tains a regulatory domain that functions in a cell- 
=-«ype and stage-specific manner. A 25-base pair 
element within this domain is necessary and suf- 
ficient for activity. This element is recognized by 
a novel transcriptional activator termed BLyF, 
whose distribution correlates with the pattern of 
expression of mb-l in B lineage cells. Thus BLyF 
may play an important role in B cell develop- 
ment, in part by regulating the activity of the 
mb-l gene. 
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