Bone Marrow Transplantation/immunology; Cyclosporine/therapeutic use; Graft vs Host Disease/prevention & control; Immunosuppressive Agents/therapeutic use; Leukemia/therapy; Neoplasm Recurrence, Local/prevention & control; Acute Disease; Adolescent; Bone Marrow Transplantation/adverse effects; Child; Child, Preschool; Cyclosporine/administration & dosage; Dose-Response Relationship, Drug; Female; Graft Survival; Graft vs Host Disease/mortality; Graft vs Leukemia Effect/drug effects; Histocompatibility; Humans; Immunosuppressive Agents/administration & dosage; Infant; Italy/epidemiology; Leukemia/mortality; Male; Neoplasm Recurrence, Local/epidemiology; Nuclear Family; Precursor Cell Lymphoblastic Leukemia-Lymphoma/mortality; Precursor Cell Lymphoblastic Leukemia-Lymphoma/therapy; Prospective Studies; Risk; Tissue Donors; Transplantation Conditioning; Treatment Outcome