1. Department of Pathology, NYU Cancer Institute, New York University School of Medicine, 550 First Avenue, MSB 599, New York, NY 10016, USA.
2. European Institute of Oncology, 435 Via Ripamonti, 20141 Milan, Italy.
3. Department of Biomedical Sciences, College of Medicine, Florida State University, Tallahassee, FL 32306, USA.
4. MRC Mammalian Genetics Unit, Harwell, Oxfordshire OX11 0RD, UK.
5. Merck Research Laboratories, 33 Avenue Louis Pasteur, Boston, MA 02115, USA.