1. Department of Molecular and Cellular Biology, Harvard University, Cambridge, Massachusetts 02138, USA
2. Current address: School of Biological Sciences, University of Utah, Salt Lake City, Utah 84112, USA
3. Center for Brain Science, Harvard University, Cambridge, Massachusetts 02138, USA
4. Broad Institute of MIT and Harvard, Cambridge, Massachusetts 02142, USA
5. Harvard Stem Cell Institute, Cambridge, Massachusetts 02138, USA
6. FAS Center for Systems Biology, Harvard University, Cambridge, Massachusetts 02138, USA