“…Furthermore, the wild-type enhancer in the MoMLV long terminal repeat (LTR) has been replaced by the enhancer of the PyF101 mutant of polyoma virus. This vector exhibited expression levels of introduced genes that equalled the levels of their endogenous counterparts in adenosine deaminase deficient T cells from a patient suffering from severe combined immunodeficiency disease (ADA − SCID) and in T cells isolated from a rhesus monkey transplanted with transduced bone marrow cells 28,29 However, when transduced bone marrow cells were transplanted into mice and 6 months later peripheral blood mononuclear cells were examined, among six mice that were positive for the provirus only two were found to express human ADA. Moreover, expression levels did not correlate with proviral copy numbers.…”