About the Authors
Department of Animal Biology, Center for Animal Transgenesis and Germ Cell Research, University of Pennsylvania School of Veterinary Medicine, Philadelphia, Pennsylvania, United States of America
The authors have declared that no competing interests exist.
Conceived and designed the experiments: JZ PJW. Performed the experiments: JZ FY NAL PJW. Analyzed the data: JZ PJW. Wrote the paper: PJW.