About the Authors
Division of Biology, California Institute of Technology, Pasadena, California, United States of America
The authors have declared that no competing interests exist.
Conceived and designed the experiments: XML LY DB. Performed the experiments: XML MYYL RAF. Analyzed the data: XML PJB LY DB. Contributed reagents/materials/analysis tools: XML APW ABB. Wrote the paper: XML APW PJB LY DB.