118 
Monitoring Stem Cell Research 
process of repeated expansion and preservation, subtle 
changes in the growth conditions or other variables may give 
rise to "selective pressures" that can increase the heterogene- 
ity in a stem cell preparation by favoring the multiplication of 
advantaged cell variants in the population. It is not known at 
present how many of the 78 human ESC preparations, desig- 
nated as eligible for federal funding under the current policy, 
have developed genetic variants that may make them unsuit- 
able for further research. 
Whether several cycles of freezing and thawing change the 
phenotypic characteristics of stem cell preparations needs de- 
tailed study. However, the practical advantages of preserving 
stem cell preparations by freezing are too large to ignore. Such 
preservation makes it possible to repeat an experiment many 
times with a very similar stem cell preparation. It would also 
make it possible, should stem cell based therapies be devel- 
oped in the future, to treat multiple patients with a common, 
well-characterized cell preparation derived from a single initial 
stem cell sample. 
D. Chromosome Changes 
In addition to the possible loss of homogeneity in stem cell 
preparations owing to variability in growth conditions or to 
freezing and thawing, there is the possibility of variation being 
introduced during the processes of growth and cell division. 
Normal human stem cells (like all human somatic cells) have 46 
chromosomes. During the copying of chromosomal DNA and 
the separation of daughter chromosomes at cell division, rare 
mistakes occur that lead to the formation of abnormal chromo- 
somes or maldistribution of normal ones. Cells with abnormal 
chromosomes or chromosome numbers can progress to malig- 
nancy, so retention of the normal human chromosome number 
and structure is an essential characteristic of useful human 
stem cell preparations. The most studied preparations of hu- 
man stem cells generally have normal human chromosome 
numbers and structure.^’^^* Nevertheless, vigilance is needed. 
* As of November 2003, reports were available eibout the chromosome pat- 
terns of only 21 out of the 78 ESC preparations designated as eligible for fed- 
eral funding; 11 of the 12 preparations currently available have had their 
PRE -PUBLICATION VERSION 
