1. Departments of Dermatology and Cell and Developmental Biology, University of Pennsylvania School of Medicine, Philadelphia, PA 19104, USA
2. Cell and Molecular Biology Graduate Group, University of Pennsylvania School of Medicine, Philadelphia, PA 19104, USA
3. Section of Endocrinology and Metabolism, Department of Internal Medicine, Yale University, New Haven, CT 06520, USA
4. Cancer and Developmental Biology Laboratory, National Cancer Institute-Frederick, Frederick, MD 21702, USA
5. Swiss Institute for Experimental Cancer Research, NCCR Molecular Oncology,CH-1066 Epalinges s/Lausanne, Switzerland
6. Laboratory of Cellular Carcinogenesis and Tumor Promotion, National Cancer Institute, Bethesda, MD 20892, USA