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for even a small number of chromosomally abnormal cells 
could end up causing cancer in future clinical trials of stem cell 
based therapies. 
E. Developmental Heterogeneity of Stem Cell Preparations 
The in vitro growth conditions and the presence of specific 
chemicals or proteins, or both, in the culture medium can influ- 
ence the differentiation pathway taken by stem cells as they 
start to differentiate. Thus, even initially homogeneous, “single 
cell cloned" stem cell preparations may become development 
tally heterogeneous over time, with respect to the percentage 
of cells in the preparation that are in one or another differenti- 
ated state. For example, a stem cell preparation after growth 
in vitro under specific conditions might contain 75 percent fully 
differentiated (insulin-producing) cells and 25 percent partially 
differentiated cells. The biological properties of the fully differ- 
entiated cells and the partially differentiated cells are likely to 
be different. If such a cell preparation is used in research, or 
transplanted into an animal model of human disease and a bio- 
logical effect is observed, one must do additional experiments 
to determine whether the effect was due to the fully differenti- 
ated cells or to the partially differentiated cells (or perhaps to 
both acting together) in the now mixed preparation. 
F, Microbial Contamination 
Stem cell preparations originally isolated from humans and 
expanded in vitro may also be variably contaminated with hu- 
man viruses , bacteria , fungi and mycoplasma . ESC prepara- 
tions isolated using mouse feeder cell layers might also be 
contaminated with mouse viruses. Specific tests need to be 
performed on the source tissue and periodically on the result- 
chromosome patterns characterized, and they appear normal. However, a 
recent publication, presenting results from two different laboratories, reports 
abnormalities in chromosome number and structure in some samples of three 
different human ESC preparations. Two of these ESC preparations are 
among the twelve preparations currently available for federal funding. 
[Draper, J.S., et al., "Recurrent gain of chromosomes 17q and 12 in cultured 
human embryonic stem cells," Nature Biotechnology December 7, 2003, ad- 
vance online publication.] 
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