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Monitoring Stem Cell Research 
ing stem cell preparations to rule out the presence of these 
contaminants. Some of these contaminants can also multiply 
when stem cells are grown in vitro, and their presence can in- 
fluence the results obtained when stem cell preparations are 
used in subsequent experiments. The presence of such con- 
taminants can also potentially affect the reproducibility of the 
results of experiments in which stem cell preparations are 
studied in vivo in experimental animals. 
In summary, there are numerous challenges to obtaining 
and preserving the uniform and stable preparations of stem 
cells necessary for reliable research and, eventually, for safe 
and effective possible therapies. Researchers must address 
multiple factors in order to maximize the probability of obtain- 
ing reproducible results with human stem cell preparations. 
Human stem cell preparations that are 
• “single cell cloned,” with a normal chromosome struc- 
ture and number, and 
• stored as multiple samples that are preserved at very 
low temperature, and 
• compared in experiments where cells from the same lot 
of frozen material are used, and 
• well-characterized as to the absence of cellular, viral, 
bacterial, fungal, and mycoplasma contaminants, and 
• tested to determine the proportion of stem cells and 
various differentiated cells in the cell preparation used 
in the experiments, 
are most likely to yield experimental results that will be repro- 
ducible. Preparations with these properties will be the most 
useful both in basic research and in investigations of possible 
clinical applications. 
PRE -PUBLICATION VERSION 
