Why are retroviruses useful as vectors for animal cells? Identify the principal advantages that make them attractive for gene delivery.

Introduction / Context:
Retroviral and lentiviral vectors are staples in animal-cell gene delivery. Their biology offers several advantages that facilitate stable and efficient gene transfer, particularly for long-term expression in dividing cells (retrovirus) or even non-dividing cells (lentivirus derivatives).

Given Data / Assumptions:

• The question addresses general advantages of retroviral systems as vectors.
• We consider common properties exploited in vector engineering.

Concept / Approach:
Engineered retroviruses often have broad tropism (host range) after pseudotyping (e.g., VSV-G), can establish persistent infection without immediate cytolysis, and use potent Long Terminal Repeat (LTR) promoters or internal promoters for robust expression. These features collectively make retroviral systems versatile for stable integration and gene expression in many animal cell types.

Step-by-Step Solution:

Verification / Alternative check:
Standard vector manuals list these benefits; clinical and research applications exploit integration for stable lines and in vivo gene transfer.

Why Other Options Are Wrong:

• Single advantages alone are incomplete; retroviruses combine several useful traits.
• “None of the above” contradicts extensive empirical use in gene delivery.

Common Pitfalls:
Ignoring biosafety modifications; modern vectors are replication-defective and split into packaging/helper components.

Final Answer:
All of the above