Control of the Immunoglobulin Heavy-Chain Gene 
ing B lymphocyte maturation. In collaboration 
with Stephen Desiderio (HHMI, Johns Hopkins 
University School of Medicine), we analyzed Id 
mRNA expression in a variety of cell lines repre- 
senting different stages of B lymphoid-cell dif- 
ferentiation. Only two of these lines, represent- 
ing the earliest stages of development, were 
Jfound to express Id. We confirmed that these 
cells were unable to support the activity of the 
IgH enhancer, whereas later-stage cells could. 
Hence Id is restricted to early B lymphoid-cell 
progenitors, and its presence correlates inversely 
with IgH enhancer activity. Presumably Id serves 
to keep enhancer activity low in the B cell pro- 
genitors. Further differentiation of B cells would 
require a decrease in Id expression that, in turn, 
would allow the IgH enhancer to become active. 
We have shown that Id also plays an important 
role in the development of another hematopoi- 
etic lineage, the myeloid lineage. In collabora- 
tion with Giovani Rovera (Wistar Institute), we 
demonstrated that Id is expressed in neutrophil 
precursors. When induced to differentiate in cul- 
ture, the Id levels in these cells were observed to 
decrease, but only transiently. Cells bearing an 
artificial, constitutively expressed Id gene failed 
to differentiate. Hence an appropriate, transi- 
ent shutoflf of Id is required for neutrophil 
development. 
An important corollary to these results is that 
myeloid cells must utilize proteins of the E2-5 
family to promote their differentiation. We are 
presently attempting to identify and characterize 
these proteins. 
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