[1] Natl Ctr Infect & Parasit Dis, Dept Virol, Lab Cell Cultures, Sofia 1233, Bulgaria
[2] Bulgarian Acad Sci, Inst Organ Chem, Ctr Phytochem, Akad G Bontchev, Sofia 1156, Bulgaria
[3] Natl Ctr Infect & Parasit Dis, Dept Virol, Retrovirus Lab, Sofia 1233, Bulgaria
[4] Bulgarian Acad Sci, Inst Expt Pathol & Parasitol, Akad G Bontchev, Sofia 1156, Bulgaria
[5] Sofia Univ St Kliment Ohridski, Fac Biol, Dept Methodol Biol Educ, Sofia, Bulgaria
[6] Int Ctr Genet Engn & Biotechnol, New Delhi, India