1. Retroelement Section, Paul-Ehrlich-Institut, Langen, Germany.
2. Skin Cancer Unit, German Cancer Research Center Heidelberg/Department of Dermatology, University Hospital of Mannheim, Mannheim, Germany.
3. Present address: Department of Dermatology, University Clinic of Würzburg, Würzburg, Germany.
4. Biostatistics section, Paul-Ehrlich-Institut, Langen, Germany.