 Design of the study Paediatric patients affected by haematological malignancies and eligible to undergo HSCT from an unrelated volunteer will be stratified according to the degree of compatibility with their donor, the source of haematopoietic stem cells employed BM vs. PB and the disease phase good vs. poor prognosis. In particular, on the basis of compatibility with their donor, patients will be allocated to 2 different arms: those transplanted from an unrelated donor either perfectly matched or with a single allelic disparity at one of the HLA loci i.e. A, B, C, and DrB1 vs. those transplanted from an unrelated donor either with 2 allelic disparities or with an antigenic disparity at the HLA loci i.e. A, B, C, and DrB1. 