a genetic hierarchy controlling B cell development. 
A major promoter of the Oct- 2 gene has been identi- 
fied and sequenced. This region is being function- 
ally analyzed. Furthermore, since the Oct- 2 gene is a 
large locus (~100 kb), it has been surveyed for 
lineage-specific DNase I-hypersensitive sites. Such 
sites may represent enhancer or locus control region 
elements. Two such sites have been localized to 
large introns separating upstream coding exons. 
These regions are being tested for function. 
Regulation of mb-1 Gene Activity 
The mb-1 gene encodes an integral membrane 
protein that appears to be required for the surface 
expression and signaling function (s) of the Ig re- 
ceptor on B lymphocytes. The gene, which is ex- 
pressed in a lineage-restricted manner, is activated 
early in B cell ontogeny, continues to be expressed 
in mature B cells, but is turned off in terminally 
differentiated plasma cells. Dr. Singh's laboratory 
has identified the mb-1 promoter and functionally 
tested its activity by transient transfections. A 737- 
bp promoter fragment preferentially stimulates ac- 
curately initiated transcription in mi>-i-expressing 
B cells. Deletion analysis of the promoter suggests 
the presence of two functional domains — proximal 
and distal. Both domains independently activate 
transcription from a heterologous promoter. The 
distal domain functions in a cell-type- and stage- 
specific manner, activating transcription in B cells 
but not in T cells or plasma cells. A 25-bp element 
within this domain is necessary and sufficient for 
activity. This element is recognized by a novel 
cell-type- and stage-specific transcription factor, 
termed BLyF. The binding of BLyP correlates with 
the ability of the regulatory element to stimulate 
transcription. Thus BLyF appears to positively regu- 
late transcription of the mb-1 gene. The results also 
suggest that the inactivity of the mb-1 locus in 
plasma cells is not simply due to the loss of BLyF 
activity. 
Dr. Singh is also Assistant Professor of Molecu- 
lar Genetics and Cell Biology at the University of 
Chicago. 
Articles 
Feldhaus, A.L., MbangkoUo, D., Arvin, K.L., 
Klug, C.A., and Singh, H. 1992. BLyF, a novel 
cell-type- and stage-specific regulator of the B- 
lymphocyte gene mb-1. Mol Cell Biol 12:1126- 
1133. 
Miller, C.L., Feldhaus, A.L., Rooney, J.W., Rhodes, 
L.D., Sibley, C.H., and Singh, H. 1991. Regula- 
tion and a possible stage-specific function of Oct- 
2 during pre-B-cell differentiation. Mol Cell Biol 
11:4885-4894. 
DEVELOPMENTAL GENETICS 
Philippe M. Soriano, Ph.D., Assistant Investigator 
Research in Dr. Soriano's laboratory is focused on 
early development of the mouse embryo, with par- 
ticular emphasis on mechanisms underlying cell- 
cell interactions and signaling. The experimental 
approach involves the derivation of mouse lines 
carrying mutations in genes implicated in these pro- 
cesses through targeted disruption or promoter 
traps in embryonic stem (ES) cells. 
Genetic Analysis 
of Protein-Tyrosine Kinases 
Previous work of this laboratory was focused on 
the derivation of mouse mutants for the c-src proto- 
oncogene. Although the oncogenic variant of the 
gene had been well characterized, little was known 
about the essential functions of the normal cellular 
counterpart. The src protein is a nonreceptor tyro- 
sine kinase highly expressed in the central nervous 
system (CNS) during embryogenesis and in many 
cell types in the adult, including cells of hematopoi- 
etic origin such as platelets. Surprisingly, disruption 
of the src gene by homologous recombination in ES 
cells did not lead to embryonic death, but instead to 
a highly restricted recessive phenotype in the adult, 
osteopetrosis. This phenotype is characterized by 
defective osteoclast function leading to a defect in 
bone remodeling. 
To characterize the mutation further; work done 
in collaboration with Dr. Greg Mundy (University of 
Texas, San Antonio) has shown that the inherent de- 
fect in src^ mice is confined to the osteoclasts and is 
cell autonomous, since the defect can be corrected 
by fetal liver transplants and is therefore indepen- 
dent of the bone marrow microenvironment. These 
studies may help to identify a substrate unique to 
src. (The project described above was supported by 
a grant from the National Institutes of Health.) 
The absence of a more severe phenotype in src~ 
GENETICS 261 
