fected |JL gene construct could be controlled, the 
ceils were allowed to establish |jl gene transcription 
in the presence of the enhancer and subsequently a 
portion of the transfected cells was allowed to de- 
lete the enhancer from the |x gene by site-specific 
D-to-J rearrangement. This experiment showed that 
deletion of the enhancer from an actively tran- 
scribed gene reproducibly resulted in switching off 
of li, gene expression. Thus the enhancer seems to 
be required for both establishment and mainte- 
nance of |JL gene expression. 
III. Isolation and Characterization of Lymphocyte- 
specific cDNA Clones. 
To identify a set of new markers for individual 
stages of the lymphocyte cell lineage and to find 
gene products that are important for the lymphoid 
cell differentiation pathway, Dr. Grosschedl and his 
colleagues isolated novel lymphocyte-specific cDNA 
clones. By subtractive hybridization of a pre-B cell 
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cDNA library with cDNA from an erythroid cell line, 
cDNAs from genes that are expressed only in cells 
of the B and T lymphocyte lineage were isolated. 
These genes show three distinct expression pat- 
terns. Some are expressed in cell lines representing 
T cells and early stage pre-B cells but are not ex- 
pressed in cells of the mature B cell stage. Another 
set of genes is expressed only in B lymphocytes, 
whereas a third set is expressed in all analyzed B 
and T cell lines. Analysis of the 5 '-terminal cDNA 
sequences indicated that five of the isolated clones 
represent previously unknown genes. The nucleo- 
tide sequences of these cDNA clones are being de- 
termined, and the cDNAs are being expressed in 
bacteria to obtain protein for the generation of an- 
tibody. 
Dr. Grosschedl is also Assistant Professor of Mi- 
crobiology and Immunology and of Biochemistry 
and Biophysics at the University of California at San 
Francisco. 
Articles 
Grosschedl, R., and Marx, M. 1988. Stable propagation of the active transcriptional state of an immunoglobu- 
lin [xgene requires continuous enhancer function. Cell 55:645-654. 
Tsukamoto, A., Grosschedl, R., Guzman, R.C., Parslow, T, and Varmus, H.E. 1988. Expression of the int-\ 
gene in transgenic mice is associated with mammary gland hyperplasia and adenocarcinomas in male and 
female mice. Cell 55:619-625. 
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