1. Research Group of Peptide Chemistry, Hungarian Academy of Sciences, H-1518 Budapest, Hungary, 1st Instute of Biochemistry, Semmelweis University Medical School, H-1444 Budapest, Hungary, Cancer Research Laboratory, University of Nottingham, Nottingham NG7 2RD, U.K., Department of Anatomy, University Medical School, H-7643 Pécs, Hungary, National Institute of Oncology, H-1122 Budapest, Hungary, and Department of Organic Chemistry, Eötvös L. University, H-1518 Budapest, Hungary