Strategies for packaging cell line construction. A generic helper virus modeled on Moloney murine leukemia virus (Mo-MLV) is shown at top. Strategies used to generate deleted viruses that provide all proteins required for viral replication but which cannot themselves replicate are shown. Deleted viruses of types A and B involve single DNA constructs, and those of type C involve two constructs. Nonviral transcription signals are shown as open boxes. The bottom construct in each set shows a generic retroviral vector. Potential mechanisms for recombination leading to helper virus production are indicated. Short arrows indicate promoters and direction of transcription.