1. University of Washington 1 Department of Biology , , Seattle, WA 98195 , USA
2. University of Washington 2 Molecular and Cellular Biology Program , , Seattle, WA 98195 , USA
3. 3 Molecular, Cell and Developmental Biology Department, University of California, Los Angeles, CA 90095, USA
4. Molecular Biology Institute, University of California 4 , Los Angeles, CA 90095 , USA
5. Institute for Stem Cell and Regenerative Medicine, University of Washington 5 , Seattle, WA 98109 , USA