Research and Therapy Developments 
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led to the isolation of an increasing number of adult (non- 
embryonic) stem cells (dashed box area of Figure 1) from such 
I tissues as bone marrow (for example, hematopoietic and mes- 
; enchvmal stem cells ), brain (for example, neural stem cells) 
I and other tissues [see (4)]. Although these stem cell prepara- 
• tions differ from one another in their future fates, they tend to 
be grouped together (especially in the public policy debates) 
under the name “adult stem cells,” even though they may have 
been obtained from children or even from umbilical cord blood 
obtained at the time of childbirth. 
I Subsequent exposure to additional differentiation signals 
! can cause these specialized stem cells to differentiate further, 
I so that they finally give rise to the variety of differentiated cells 
that make up the adult body (labeled A-D in Figure 1). At each 
stage of the differentiation process, specific sets of genes are 
expressed (or "turned on”) and other sets are repressed (or ; 
“turned off”), to produce the specific proteins that give each 
I cell its distinctive properties. At each stage along the way, 
proteins called transcription factors play key roles in determin- 
ing which sets of genes are expressed and repressed, and 
therefore what sort of a cell the newly differentiated cell will ] 
become. * 
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n. REPRODUCIBLE RESULTS USING STEM CELL ! 
PREPARATIONS AND THEIR DERIVATIVES 
A major goal of scientific research is the acquisition of reli- 
able knowledge based on experiments that yield reproducible 
I results. Reproducible results are possible only if the materials 
I used in experiments remain constant and stable. To obtain re- 
j producible results in experiments using stem cells, it is essen- 
tial to produce, preserve, characterize, and continually re- 
,1 characterize preparations of stem cells in ways that increase 
the likelihood that the cells used to repeat experiments will 
remain unchanged — a technically challenging task. The ten- 
dency of stem cells in vitro to differentiate spontaneously into 
more specialized cells makes the task of obtaining homogene- 
ous and stable stem cell preparations especially challenging, 
and much basic research is needed to learn how to control the 
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PRE -PUBLICATION VERSION 
