Expression systems are DNA-encoded genetic constructions that are designed to create a protein or RNA (ribonucleic acid) inside or outside of a cell. Expression systems are utilised in both research and commercial enzyme and pharmaceutical manufacturing. Proteins are expressed using expression systems in a variety of ways, from structural biology to in vivo research or as medicinal agents. An expression system is made up of three components that are required for the synthesis of recombinant proteins. Firstly, energy and machinery for protein synthesis are provided by a biological environment, most commonly a cell. Cell extracts containing the required components, i.e., cell-free protein expression systems, can also be used. Second, a vector that allows genetic material to be introduced into a cell; it contains regulatory sections that allow the genetic material to be replicated and, in most cases, selection markers for maintenance. The expression cassette, which is integrated into the vector and contains the open reading frame encoding the amino acid sequence for the protein to be expressed, is the final step.