 A variety of systems, such as inducible promoters and specific cell-signaling factors, are available to help express the protein of interest at high levels.Large quantities of a protein can then be extracted from the bacterial or eukaryotic cell.The protein can be tested for enzymatic activity under a variety of situations, the protein may be crystallized so its tertiary structure can be studied, or, in the pharmaceutical industry, the activity of new drugs against the protein can be studied.