C. HESDORFFER 4/93 
vector have a 1-2 log increase in MDR activity (32,47). In 
addition, a significant number of bone marrow cells from live 
mice transduced with MDR virus express high levels of MDR eight 
months posttransplantation. To rule out the possibility that 
long-lived lymphocytes are the source of the MDR positive cells, 
cell gating on the basis of size and morphology was used to 
exclude lymphocytes from the analysis (32). Approximately 14% of 
the granulocyte-macrophage cells in the bone marrow of a mouse 
eight months posttransplantation contain markedly increased 
amounts of MDR p-glycoprotein on their surface as compared to 
controls (32) . These data show that MDR-positive cells in this 
granulocyte-macrophage population are derived from bone marrow 
stem cells. 
We also showed that we can select in vivo for MDR-transduced cells 
in bone marrow (32) . Four transduced mice, initially positive by 
PCR for the MDR gene, subsequently lost their MDR PCR signal at 
eight months posttransplantation. These mice were given 140 /j,g 
of taxol in an attempt to enrich for bone marrow cells containing 
and expressing the human MDR gene, and to prove that MDR 
expression allows cell selection. Seven days after a single dose 
of taxol, the PCR signal in all four mice reappeared; in 
addition, FACS analysis of the peripheral blood of two of these 
mice showed 5-8% MDR-positive granulocytes (32). 
We have also recently transferred the human MDR gene into live 
mice using MDR-containing cell-free supernatants from retroviral 
amphotropic producer cells. Thus, we have demonstrated that: 
1) Bone marrow stem cells can be stably transduced with the 
human MDR gene for long periods of time; and 2) MDR-transduced 
cells are protected from taxol toxicity and can be selected. In 
these studies, PCR analysis is used to detect MDR DNA in tail 
vein blood and bone marrow (43) . Southern blot analysis also 
measures MDR DNA (89,90), and Northern blotting and SI nuclease 
analyses quantitate MDR RNA (89,91-93). In MELC transduced with 
MDR virus-containing producer cells, we show positive Southern 
and Northern blots using human MDR-specific probes (47) . Thus, 
we have established conditions for the analysis of MDR DNA and 
mRNA by Southern and Northern blotting, and for the presence and 
quantitation of MDR protein by FACS analysis. We have also 
demonstrated high-level expression of a stably transduced human 
MDR gene in 3T3 cells, MELC, and the bone marrow cells of live 
mice both short- and long-term after transplantation with MDR- 
transduced cells (32,47). 
Human bone marrow stem cells (or early precursor cells) have 
rhodaminel23Low (Rho Low ) staining, and have high levels of MDR 
expression (94) ; this Rho Low staining is probably due to the 
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