In gene therapy, an "expression cassette" is the complete package of genetic instructions packed into the viral vector. It's not just the MEF2C gene itself, it also includes:
- A promoter (the "start here" signal telling cells when to read the gene)
- Enhancers (the "read louder" signals)
- Polyadenylation signals (the "stop here" signal)
- The coding sequence (the actual MEF2C gene instructions)
Analogy: If the MEF2C gene is a recipe, the expression cassette is the whole cookbook page — the recipe itself plus all the instructions for "cook at 350°F for 30 minutes" and "serve with rice." Without the surrounding elements, the gene alone can't be properly read by cells.