Purified proteins have been used as human medicines for many decades. Before recombinant DNA technology was developed, hormones (e.g., insulin and human growth hormone) and other proteins were extracted and purified from blood and other tissues. Because of the low levels of the protein of interest in the tissues, the purification was technically very difficult and often it was more of an “enrichment” than a purification to homogeneity. Since the 1970s, the ability to genetically engineer Escherichia coli and other organisms to produce large quantities of therapeutically interesting molecules has led to the development of dozens of therapeutic proteins (1). Some of these proteins (e.g., erythropoetin) are hugely successful drugs with annual worldwide sales of several billion dollars (2).