 Experimental systems are the core of Hans-Jörg Rheinberger’s (1997) account of mid-20th Century research on protein synthesis. Rheinberger characterizes experimental systems as “systems of manipulation designed to give unknown answers to questions that the experimenters themselves are not yet clearly to ask” and also as “the smallest integral working units of research” (Rheinberger 1997, 28). He argues that research in experimental biology always “begins with the choice of a system rather than with the choice of a theoretical framework” (p. 25). He then follows a certain experimental system through time and shows how it exerted a strong effect on the development of biology. The system in question is a so-called in-vitro system for protein synthesis that was developed in the laboratory of Paul Zamecnik at Massachusetts General Hospital in the 1950s. “In vitro” means that the system doesn’t contain any living cells. It is rather based on a cell extract, originally from bacteria, later also from other organisms including rabbits and yeast. The cell extract is prepared in a way that preserves functionality of the protein synthesis machinery. Instead of the RNAs that naturally occur in the source organism, the in-vitro system may be “programmed” with exogenous or even artificial RNA. Furthermore, the experimental system contains measurement procedures (sometimes called “assays” in experimental biology) for protein synthesis activity. One such method is to measure incorporation of radioactivity introduced by amino acids containing a radioisotope such as sulfur-35, but there were other methods as well. Probably the most famous experiment done with such a system is the experiment by Marshall Nirenberg and Heinrich Matthaei (1961). These researchers added an artificial RNA, namely poly-U (U or uracil is one of the four bases of RNA) to the in-vitro system and showed that it produced a polypeptide (=protein) containing only the amino acid phenylalanine. Thus, they “cracked” the first “word” of the genetic code, namely UUU for phenylalanine.