viral gag sequences downstream from the 5' viral LTR. The Infection protocol 
used in the work of Keller et al . (1985) was very similar to that of Dick et 
al . (1985). including pretreatment of donor mice with 5-FU several days prior 
to the collection of bone marrow and a 48 hour preselection in G418 of infected 
cells before transplant into irradiated mice. Without preselection in liquid 
culture. 10-20% of the colony forming cells were G418-resistant whereas the 
percentage of resistant colonies in methyl cellulose culture increased to 
60-95% if the cells were initially preselected for 48 hours in liquid culture 
containing G418. 
When mice were transplanted with N3-infected and preselected bone marrow 
cells, 70% of the colony forming cells (CFC) from the spleens of 12 day 
reconstituted mice were resistant to G418 compared to only 5% resistant CFC 
from the spleens of mice receiving transplants of unselected bone marrow cells. 
NPT activity was also much greater in spleen cell lysates of mice receiving 
cells preselected in G418 indicating that these mice underwent reconstitution 
with a population of hematopoietic precursor cells enriched for expression of 
D 
the introduced Neo gene. The expressing cells retained the capacity for 
D 
self-renewal by reconstituting secondary recipients expressing the Neo gene. 
The proportion of G418-resistant CFC decreased with the length of time after 
reconstitution ranging from 37% resistant colonies at 6 weeks to 1% resistant 
colonies at 17 weeks post-transplant. A corresponding decrease in 
G418-resistant CFC was observed when secondary animals were reconstituted with 
bone marrow cells removed from a primary transplanted animal. In addition to 
G418-resistant CFC. NPT activity was assayed directly and was present in the 
spleen, bone marrow and thymus of several primary and secondary reconstituted 
mice. 
In a second study using the same N2 vector that was done in our laboratory 
[44] 
Recombinant DNA Research, Volume 12 
