Department of Molecular Biology, IRIS, Chiron S.p.A., Via Fiorentina 1, 53100 Siena, , Italy1
Department of Biology, University of Bologna, Via Selmi 3, 40126 Bologna, , Italy2
For correspondence. Tel: +39 0577 243565. Fax: +39 0577 243564. e-mail: enzo.scarlato{at}chiron.it