The DNA replication process of bacteriophage f1 involves specific signals for starting and stopping the synthesis of viral DNA. Mutational analysis revealed two key regions in the DNA sequence: a core region essential for DNA synthesis and containing signals for initiation and termination, and a secondary region needed only for DNA initiation. Disrupting the secondary region reduces replication activity significantly. This study sheds light on how the f1 origin of replication functions and why it can tolerate interruptions by foreign DNA without major impact on replication.