1. Division of Pediatrics and Children's Cancer Hospital, U.T. M.D. Anderson Cancer Center, Houston, Texas, USA
2. Present address: 2903 White Rock Drive, Austin, TX 78757-4448, USA
3. Department of Pediatrics, L'Institut Gustave Roussy, 39 Rue Camille Desmoulins, Villejuif F-94800, France
4. Department of Pathology, University of Oklahoma Health Sciences Center, 940 Stanton L. Young Blvd, Room 451, Oklahoma City, OK 73104, USA