In vitro transcription/translation systems are becoming more useful to produce proteins and determine the protein function. Expression of proteins in vitro provides a rapid and controllable system to express proteins. The ability to control the system by addition of other compounds has allowed for the production of proteins that had been difficult to produce by conventional expression systems. More recently, the development of continuous in vitro systems has allowed the production of larger quantities of proteins for downstream purification techniques. Also, the use of different cell lysates allows for specific post-translational modifications of the synthesized proteins. The ability to label the translated proteins can aid structural analysis and allow tracking of specific proteins.