A viral vector experiment can fail without any visible sign that something went wrong. The vector is produced correctly and injected correctly. The animals survive, routine histology looks normal, and PCR confirms vector genome delivery. The biological question is still unanswered, because the target cells were not efficiently transduced or did not express the transgene at a meaningful level.

Failures of this kind usually trace back to the construct, not the procedure. A cassette that exceeded packaging capacity and was packaged incompletely. A capsid chosen from a published result obtained in a different strain, species, or delivery route. A promoter that drove strong early expression and then silenced before the endpoint. None of these are visible at the bench until the experiment is over, and by then the cost is a full cohort and a season of work.

This white paper covers the biology of AAV and adenoviral vectors, then examines the three design-stage failure modes that account for most lost experiments: payload overflow, serotype mismatch, and promoter mismatch. The final section walks through a seven-step design workflow, with tips at each step covering component selection, substitution options when a cassette runs over, and how to check a design against the intended duration and delivery route before committing to production.