Studies of Blood Cell Formation 
poietic cells in adult animals. However, less is 
known about the role of these factors in normal 
functioning of the hematopoietic system during 
fetal development. Through use of ES cell lines 
that have been programmed to make large 
amou«ts of these growth factors during the fetal 
yolk sac phase of embryo development, new in- 
sights into the control of blood formation in the 
embryo may be gained. 
Gene Transfer and Somatic Gene Therapy 
For several years we have been studying the use 
of gene transfer to treat human genetic diseases. 
Slow but steady progress has been made, using 
viral vehicles (called vectors) to transfer a 
correcting gene into hematopoietic stem cells. 
The laboratory has studied the transfer of adeno- 
sine deaminase (ADA) , a lack of which can lead to 
a fatal disease called severe combined immunode- 
ficiency (SCID) . Jane Apperley and Barry Luskey, 
in a continuing collaboration with Stuart Orkin 
(HHMI, at the Children's Hospital, Boston), has 
demonstrated that ADA can be transferred into he- 
matopoietic stem cells. Once these cells are 
transplanted into irradiated mice, the human ADA 
gene expresses functional protein at nearly the 
levels of the endogenous mouse ADA gene. Such 
levels are sustained in 75 percent of the animals 
for longer than six months, demonstrating stable 
expression of the introduced gene after complete 
reconstitution of the hematopoietic system by the 
transplanted cells. This work demonstrates that 
the introduction and expression of a vector-borne 
gene appears to be a viable approach to the cor- 
rection of some severe diseases of the hematopoi- 
etic system in humans. 
In addition, Barry Luskey, in collaboration with 
Arthur Neinhuis and David Bodine at the National 
Institutes of Health, has begun studies aimed at 
transferring the ADA gene into monkey bone 
marrow cells. These studies are very important 
prior to any use of retroviral technology to treat 
human disease. 
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