1. Department of Medical Ethics, Philadelphia, PA 19104, USA.
2. Institute for Regenerative Medicine, Philadelphia, PA 19104, USA.
3. Department of Cell and Developmental Biology, Philadelphia, PA 19104, USA.
4. Department of Religious Studies and Feinberg School of Medicine, Northwestern University, Chicago, IL 60611, USA.
5. Department of Medicine, Perelman School of Medicine, University of Pennsylvania, Philadelphia, PA 19104, USA.