I propose to use the LMO1 transgenic model to study retroviral vector insertional mutagenesis in hematopoietic stem cells. The sensitivity of LMO1 mice to acquisition of secondary mutations for leukemic transformation will allow for the characterization of insertional events promoting clonal outgrowth and eventual transformation, which occur too infrequently in current animal models for significant study. The outcome of tumor formation in this model allows for the important distinction between transforming insertional events and the more frequent non-transforming events, for focused study of the frequency and preferred gene targets of the former. These studies will also examine the effects of drug selection on clonal outgrowth and transformation, which impacts the use of in vivo selection for enrichment of transduced hematopoietic stem cells. These concepts have direct bearing on the safe application of hematopoietic stem cell gene therapy, and could provide an increased understanding of mechanisms of hematopoietic transformation. Further, the insertion sites may offer insight into the cooperativity of transforming events, while the LMO1 mouse might be explored as a screening tool for other gene transfer vectors. ? ? ?