Submitted March 6, 2003
Accepted April 14, 2003
Long targeting arms do not increase the efficiency of homologous recombination in the 
-globin locus of murine embryonic stem cells
Zhi Hong Lu, Jason T Books, Richard M Kaufman, and Timothy J Ley*
Section of Stem Cell Biology, Division of Oncology, Washington University, St. Louis, MO, USA
Harvard Medical School, Department of Pathology, Brigham and Women's Hospital, Boston, MA, USA
* Corresponding author; email: tley{at}im.wustl.edu.
The correction of mutant 
-globin genes has long been a therapeutic goal for patients with -thalassemia or hemoglobinopathies. The use of homologous recombination (HR) to achieve this goal is an attractive approach, since it eliminates the need to include regulatory sequences in the therapeutic construct, and it eliminates mutagenesis induced by random integration. However, HR is a very inefficient process for gene correction, and its efficiency is probably locus dependent. The length of targeting arms is thought to be a determinant of targeting efficiency, so we compared the ability of standard (8 Kb) vs. very long (16, 24, and 110 Kb) regions of homology to correct a mutant murine -globin gene in embryonic stem cells. Increasing the length of the targeting sequences did not increase the efficiency of HR in this locus, suggesting that alternative approaches will be required to improve the efficiency of this approach for globin gene correction.
