FIG. 7.

Effect of the −21/−28 T-stretch on helper functions. Sequences of the wild type and mutant are compared at the top. The T stretch is boxed in black, and differences from wild type are shown in boldface Ψ⁻ proviral clones containing a wild-type T stretch or the −21/−28 mutation were cotransfected with a puromycin resistance-conferring retroviral vector. Virus was collected, quantified by RT activity, and used to infect NIH 3T3 cells. Shown is the average number of puromycin-resistant colonies per unit of RT for three independent experiments.