1. Howard Hughes Medical Institute, Life Sciences Institute, Department of Internal Medicine, and Center for Stem Cell Biology, University of Michigan, Ann Arbor, MI 48109–2216, USA.
2. Children’s Research Institute, Department of Pediatrics, University of Texas Southwestern Medical Center, Dallas, TX 75390, USA.
3. Department of Pathology, University of Michigan, Ann Arbor, MI 48109–2216, USA.
4. Department of Dermatology, University of Michigan, Ann Arbor, MI 48109–2216, USA.