Infection of quiescent CV-1 cells with simian virus 40 mutant tsA30 at 37°C resulted in the induction of two rounds of cellular DNA synthesis in T-antigen-positive cells, as previously described for wild-type simian virus 40. Following infection with tsA30 at 40.5°C, T-antigen-positive cells were induced into S phase and reached a diploid G2 DNA content; however, a second S phase was not initiated. The failure of tsA30-infected CV-1 cells to enter tetraploid S phase at 40.5°C identifies a

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... gen function, distinct from T-antigen functions responsible for stimulation of cell DNA synthesis, which is required for initiation of a second round of DNA synthesis without mitosis.