1. Department of Oncology, Hospital Infantil de M�xico Federico G�mez, Mexico City 06720, Mexico
2. Department of Pediatric Oncology, Instituto Nacional de Pediatr�a, Mexico City 04530, Mexico
3. Facultad de Estudios Superiores Zaragoza, Universidad Nacional Aut�noma de M�xico, Mexico City 09230, Mexico
4. Department of Hematology, Hospital Infantil de M�xico Federico G�mez, Mexico City 06720, Mexico