FIG. 6.

Amplification of a fragment linking endogenous polytropic retroviral sequences and 4070A env. Primer HC4 identifies with the endogenous polytropic retroviral sequence, while RV170 lies in 4070A env. A signal was obtained with DNA from infected NIH 3T3 cells. No amplification was achieved in reactions with DNA from parental NIH 3T3 cells, GP+envAM12 cells, and an early stock of GP+envAM12/pBabeNeo producer cells. As a positive control, plasmid pCRII-c3 was used, which contains the cloned PCR-amplified fragment linking the endogenous ecotropic sequence to 4070A env, as obtained from the reaction in Fig. 3c.