“…The successful application of retroviruses as gene transfer vectors in the mammalian hematopoietic system is dependent on efficient infection by the retrovirus as well as stable integration and adequate expression of an unrearranged provirus. In vitro, retroviruses have been used to reliably transfer and express exogenous genes in a variety of hematopoietic cells, including undifferentiated progenitor cell lines (26,27,29). However, in undifferentiated hematopoietic cells in vivo, the utility of retroviral vectors in a number of investigations has been impaired by low levels of expression and/or lack of sustained expression of exogenous genes from viral long terminal repeats (LTRs) (2,9,16,24,35,56).…”