Non-random chromosomal translocations can be found in about half of acute leukaemia patients and mostly lead to either over-expression of proto-oncogenes or creation of novel fusion genes. To assess the oncogenic potential and characterize the underlying mechanisms mediated by these candidate oncoproteins, a retroviral transduction/transformation assay (RTTA) has been successfully employed to study the biological impacts of a number of proto-oncoproteins and novel fusion proteins in primary hematopoietic cells both in vitro and in vivo. To further widen the application of the RTTA, a lentiviral transduction/transformation assay (LTTA) has also been developed to target the most quiescent hematopoietic stem cells (HSCs). This chapter will cover both the RTTA and LTTA for studying candidate oncogenes involved in human leukaemia.