Nature 436, 1035–1039 (2005)

In this Letter, the surname of Bettina Schmid was incorrectly spelled as Schmidt. In addition, her affiliation should have been the Adolf-Butenandt-Institute, Department of Biochemistry, Schillerstrasse 44, D-80336 Munich, Germany, not her previous affiliation (the Stem Cell Program and Division Hematology/Oncology Children's Hospital and Dana-Farber Cancer Institute) as published.