128 
Monitoring Stem Cell Research 
task. And clinical uses might require a separate FDA approval 
for every single cloned stem cell line or its derivatives. 
The ability to produce cloned mouse stem cells and geneti- 
cally modify them in vitro has made possible an experiment 
demonstrating the potential of cloned human embryonic stem 
cells in the possible future treatment of human genetic dis- 
eases. Rideout et al.^^ used a mutant mouse strain that was 
deficient in immune system function. They produced a cloned 
mouse embryonic stem cell line carrying the mutation, and 
then specifically repaired that gene mutation in vitro. The re- 
paired cloned stem cell preparation was then differentiated in 
vitro into bone marrow precursor cells. When these precursor 
cells were injected back into the genetically mutant mice, they 
produced partial restoration of immune system function. 
Production of cloned human embryonic stem cell prepara- 
tions remains technically very difficult and ethically controver- 
sial. Recently however, Chen and coworkers^^ have reported 
that fusion of human fibroblasts with enucleated rabbit oo- 
cytes in vitro leads to the development of embryo-like struc- 
tures from which cell preparations with properties similar to 
human embryonic stem cells can be isolated. This work needs 
to be confirmed by repetition in other laboratories. 
In addition, further work is needed to decisively settle the 
question of whether rabbit (or human egg donor) mitochon- 
drial DNA and rabbit (or human egg donor) mitochondrial pro- 
teins persist in the embryonic stem cell preparations. Persis- 
tence of these foreign mitochondrial proteins in these human 
E SC-like preparations could possibly increase the probability 
of immune rejection of the cloned cells, thus limiting their 
clinical application, although the immune reaction might not 
be as severe as that to foreign proteins produced under the 
direction of chromosomal genes. The presence of foreign or 
aberrant mitochondria also carries the risk of transmitting mi- 
tochondrial disease (caused by defects in mitochondrial DNA) 
that could be detrimental to the cells and to the recipient into 
whom they might eventually be transplanted. 
PRE -PUBLICATION VERSION 
