TABLE 1.

The titer of vector produced by transient transfection of ENTV Env expression plasmids is much lower than that of JSRV Env plasmids^a

Transfected env plasmid	Titer of LAPSN vector (AP+ foci/ml) measured on SSF cells
None	<2
pCSI-Jenv	3 × 103
pCMV3JS21ΔGP	4 × 103
pCSI-Eenv	10
pCMV3ENTVΔGP	<2
pMDG (VSV-G)	5 × 103

^aVectors were produced by transient transfection of the indicated plasmids into NIH 3T3 cells that make MoMLV Gag-Pol proteins and that contain the LAPSN vector, as described in Materials and Methods. Plasmids pCMV3JS21ΔGP and pCMV3ENTVΔGP have been previously described (1). Titers of viral stocks were determined by infection of sheep skin fibroblast (SSF) cells seeded the day before at 10⁵ cells per well (diameter, 3.5 cm) of 6-well plates. AP⁺ foci were counted 3 days after vector exposure. Results are means of two experiments. Experimental values varied by up to four-fold from the mean.