An expression cassette is a part of a vector DNA used for cloning and transformation. In each successful transformation, the expression cassette directs the cell's machinery to make RNA and protein. Some expression cassettes are designed for modular cloning of protein-encoding sequences so that the same cassette can easily be altered to make different proteins.
An expression cassette is composed of one or more genes and the sequences controlling their expression. Three components comprise an expression cassette: a promoter sequence, an open reading frame, and a 3' untranslated region that, in eukaryotes, usually contains a polyadenylation site.
Different expression cassettes can be transformed into different organisms including bacteria, yeast, plants, and mammalian cells as long as the correct regulatory sequences are used.