pro-B cells that have not yet undergone IgH gene 
rearrangement. All of these cell lines express sterile 
H transcripts, indicating that the IgH enhancer is ac- 
tive. Only two cell lines were found to express Id, 
and these do not express sterile n transcripts. Hence 
an inverse correlation exists between Id expression 
and enhancer activity. What remains to be deter- 
mined, however, is whether ectopic expression 
of Id in such cells can block the differentiation 
process. 
A Role for Helix-Loop-Helix Proteins 
in Myeloid Differentiation 
To explore the role of helix-loop-helix proteins 
in other hematopoietic cell types, Dr. Kadesch and 
his colleagues used the cell line 3 2D. These cells 
are interleukin-3 (IL-3)-dependent myeloblasts 
that, on exposure to granulocyte colony-stimulating 
factor (GCSF), differentiate to mature neutrophilic 
granulocytes. In collaboration with Dr. Giovani Ro- 
vera (Wistar Institute), the laboratory showed that 
these cells required the activity of bHLH proteins to 
differentiate. This was based on the following obser- 
vations. 1) Id mRNA levels in these cells drop upon 
GCSF treatment (to 10% pretreatment levels) and 
then rise (to 80-100% pretreatment levels). 2) Pro- 
teins capable of binding an E-box probe appear in 
32DC13 nuclear extracts only after Id mRNA levels 
drop, and then this DNA-binding activity disappears 
after Id mRNA levels subsequently rise. 3) Forced 
expression of Id in these cells (using an Id cDNA) 
abolishes the appearance of the E-box-binding activ- 
ity, and the cells do not differentiate; they undergo 
apparent apoptosis upon treatment with GCSF. 
Although the late up-regulation of Id in 3 2D cells 
clearly distinguishes them from B cells (Id is stably 
shut off during B cell development) , it is clear that 
the actions of bHLH proteins are required in both 
cell types. Questions concerning the identity of the 
induced E-box-binding activity in 3 2D cells and the 
consequences of Id up-regulation later in develop- 
ment are currently being addressed. The work on 
3 2D cells was carried out with funds from Dr. Ro- 
vera's research grants and was not supported di- 
rectly by HHMI. 
Dr. Kadesch is also Associate Professor of Hu- 
man Genetics at the University of Pennsylvania 
School of Medicine. 
Articles 
Henthorn, P.S., Stewart, C.C., Kadesch, T., and 
Puck, J. M. 1991 . The gene encoding human TFE3, 
a transcription factor that binds the immunoglob- 
ulin heavy-chain enhancer, maps to Xpl 1 .22. Ge- 
nomics 11:374-378. 
Kadesch, T. 1992. Helix-loop-helix proteins in the 
regulation of immunoglobulin gene transcrip- 
tion. Immunol Today 13:31-36. 
Kreider, B.L., Benezra, R., Rovera, G., and Kadesch, 
T. 1992. Inhibition of myeloid differentiation 
by the helix-loop-helix protein Id. Science 
255:1700-1702. 
Schindler, U., Terzaghi, W., Beckmann, H., Ka- 
desch, T., and Cashmore, A.R. 1992. DNA bind- 
ing site preferences and transcriptional activation 
properties of the Arabidopsis transcription factor 
GBFl. EMBO / 11:1275-1289. 
Wilson, R.B., Kiledjian, M., Shen, C P., Benezra, R., 
ZwoUo, P., Dymecki, S.M., Desiderio, S.V., and 
Kadesch, T. 1991. Repression of immunoglobu- 
lin enhancers by the helix-loop-helix protein Id: 
implications for B-lymphoid-cell development. 
Mol Cell Biol 11:6185-6191. 
GENETIC DISORDERS OF ERYTHROPOIESIS 
YuetWaiKan, M.D., Investigator 
Dr. Kan's laboratory has been investigating the 
molecular basis of genetic disorders affecting the 
red cells, with primary emphasis on sickle cell ane- 
mia, thalassemia, and hereditary hemolytic anemia 
due to membrane defects. The pathophysiological 
mechanisms of these diseases are being studied, 
newer methods of detecting the defects designed, 
and approaches for treatment investigated. 
Detection of Sickle Cell Anemia 
and Thalassemia 
An objective of this laboratory is to devise simple, 
rapid, nonradioactive tests for the hemoglobinopa- 
thies and thalassemia, which are prevalent in many 
developing countries. Methods that have been used 
include denaturing gradient gel electrophoresis to 
detect unknown mutations in the globin gene, and 
GENETICS 205 
