1 Medical Scientist Training Program, Perelman School of Medicine, Department of Biomedical Sciences, School of Veterinary Medicine, University of Pennsylvania, Philadelphia, Pennsylvania.
2 Department of Biomedical Sciences, School of Veterinary Medicine, University of Pennsylvania, Philadelphia, Pennsylvania.
3 Department of Medicine, Perelman School of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania.
4 Department of Biomedical Sciences, School of Veterinary Medicine, Department of Cell and Developmental Biology, Perelman School of Medicine, Institute for Regenerative Medicine, University of Pennsylvania, Philadelphia, Pennsylvania. Electronic address: lengner@vet.upenn.edu.