The simian virus (SV40) genome comprises two transcription units containing the early genes and, in the other orientation, the late genes. Before viral DNA replication starts, transcription is mostly of the early genes. After the viral DNA begins to replicate, the late promoter is activated, and transcription is predominantly of the late genes (De Pamphilis and Wassarman 1982; Tooze 1980). The SV40 early gene product, large T antigen, is responsible for repression of early transcription and, indirectly or directly, for the increase in late transcription (Brady et al. 1984; Hartzell et al. 1984; Keller and Alwine 1984).