Affiliation: | (1) Laboratory of Cell Propagation, National Institute for Medical Research, The Ridgeway, NW7 1AA Mill Hill, London, UK;(2) Dipartimeno di Genetica e di Biologia dei Microrganismi, Universita di Milano, via Celoria 26, I-20133 Milano, Italy;(3) Present address: Department of Genetics, Glaxo Group Research, Greenford Road, UB6 OHE Greenford, Middlesex, UK |