A transgene is a gene or genetic sequence that has been introduced into a host cell, organism, or viral vector from an external source — typically encoding a therapeutic protein, functional gene, or regulatory RNA — to achieve a desired biological effect or therapeutic outcome. In gene therapy, the transgene constitutes the therapeutic payload delivered by a viral vector — such as AAV, lentiviral, or adenoviral vectors — and its expression level, duration, tissue specificity, and safety profile are critical determinants of clinical efficacy. Key design elements of a transgene expression cassette include the promoter, coding sequence, untranslated regions (UTRs), and polyadenylation signal, all of which are carefully optimized during process development.
