FIG. 4.

Colony-forming assays in NIH 3T3 cells. The kidney cell line CV-1 was infected with the indicated viruses, and the vaccinia virus-free supernatants were used to perform colony-forming assays in NIH 3T3 cells with the transduced neomycin resistance marker. Colonies were selected at 500 μg of the antibiotic G418 per ml and stained with crystal violet after 2 weeks. (A) Supernatants of CV-1 cells (the 1:1,000 dilution is shown) infected with the vaccinia virus triple virus gave rise to G418-resistant NIH 3T3 colonies. (B) Supernatants of the same cell line infected with wild-type virus did not result in colonies of the indicator cell line.