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bryonic stem cell preparations do not contain any mouse 
feeder cells is through “single cell cloning" (see below). 
B. Genetically Homogenous Stem Cells through Single Cell 
Cloning 
Some preparations of stem cells growing in vitro have been 
“single cell cloned," that is, grown as a population derived 
from a single stem cell. By placing a cylinder over a single cell 
located with a microscope, scientists are able to isolate within 
the cylinder all the progeny produced by subsequent cell divi- 
sions beginning from this single cell. The result is a stem cell 
preparation in which all the cells are descended from the 
original single cell. The cells within the cylinder are then har- 
vested and grown to greater numbers in vitro, and the result- 
ing stem cell preparation is said to be “single cell cloned." The 
stem cells within a “single cell cloned" population are, at least 
to begin with, genetically homogeneous because they are all 
derived from the same original cell. Some of the ESC prepara- 
tions produced prior to August 9, 2001 have been “single cell 
cloned. "^2 
C. Expansion in Vitro, Preservation, and Storage 
Reproducible results require that preparations of stem cells, 
even if genetically homogenous when first isolated, remain 
stable over time and during preservation. This, too, is not a 
simple matter with stem cells, despite the fact that the self- 
renewal characteristic of human embryonic and adult stem 
cells enables them — ^unlike differentiated cells from many hu- 
man tissues — ^to be grown in large numbers in vitro while 
maintaining their essential stem cell characteristics. After 
such expansion, many, presumably identical, vials of the cells 
can be frozen and preserved at very low temperatures. Frozen 
stem cell preparations can later be thawed and grown again in 
vitro to produce larger numbers of cells. 
As with all dividing cells, stem cells are subject to a very 
small but definite chance of mutation during DNA replication; 
thus, prolonged growth in vitro could introduce genetic hetero- 
geneity into an originally homogeneous population. During this 
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