Allogenic hematopoietic stem cell transplant (allo-HSCT) is a last therapeutic option in many cases of pediatric hematological malignancies. Hematopoietic stem cells (HSCs) can be collected from different sources. The most common sources are bone marrow (BM) and umbilical cord blood. Despite recent advances in the field, allo-HSCT is associated to a series of side-effects that severely compromise the overall survival and quality of life of recipient patients. Furthermore, the clonal dynamics by which donor HSCs regenerate the recipient’s blood and the consequences of transplantation on HSC ageing remain elusive. The ultimate goal of my project is to gain fundamental insight into clonal dynamics upon transplantation and associated damage at the single cell level.