1. Department of Cell Differentiation, Institute of Molecular Genetics, Academy of Sciences of the Czech Republic v.v.i., Prague, Czech Republic;
2. Department of Cellular and Molecular Medicine, and
3. Section of Cell and Developmental Biology, University of California, San Diego, La Jolla, CA; and
4. Stem Cell Program and Division of Hematology/Oncology, Children's Hospital and Dana-Farber Cancer Institute, Howard Hughes Medical Institute, Harvard Stem Cell Institute, Harvard Medical School, Boston, MA