1. Hematopoiesis Section, Genetics and Molecular Biology Branch, National Human Genome Research Institute (NHGRI), Bethesda, MD;
2. Inserm U1009, Villejuif, France;
3. Hematology Laboratory, R. Debré Hospital, Paris, France;
4. Paris VII–Denis Diderot University, Paris, France;
5. New York Blood Center, New York, NY; and
6. Embryonic Stem Cell and Transgenic Mouse Core Facility, NHGRI, Bethesda, MD