1. G. J. Hannon is at Cold Spring Harbor Laboratory, 1 Bungtown Road, Cold Spring Harbor, NY 11724, and Genetica Inc. Post Office Box 99, Cold Spring Harbor, NY 11724, USA.
2. P. Sun and D. S. Conklin are at Cold Spring Harbor Laboratory, 1 Bungtown Road, Cold Spring Harbor, NY 11724, USA.
3. A. Carnero is at Cold Spring Harbor Laboratory, 1 Bungtown Road, Cold Spring Harbor, NY 11724, USA, and The Institute of Child Health, 30 Guilford Street, London WC1N 1EH, UK.
4. L. Y. Xie is at Genetica Inc. Post Office Box 99, Cold Spring Harbor, NY 11724, USA.
5. R. Maestro is at Cold Spring Harbor Laboratory, 1 Bungtown Road, Cold Spring Harbor, NY 11724, USA, and Centro di Riferimento Oncologico, 33081 Aviano, Italy.
6. D. Beach is at The Institute of Child Health, 30 Guilford Street, London WC1N 1EH, UK, and Genetica Inc. Post Office Box 99, Cold Spring Harbor, NY 11724, USA.