Lysogenic bacteria, or virus-infected bacteria, were the primary experimental models used by scientists working in the laboratories of the Pasteur Institute in Paris, France, during the 1950s and 1960s. Historians of science have noted that the use of lysogenic bacteria as a model in microbiological research influenced the scientific achievements of the Pasteur Institute's scientists. Francois Jacob and Jacques Monod used lysogenic bacteria to develop their operon model of gene regulation, to investigate the cellular regulatory mechanisms of the lysogenic life cycle, and to infer the process of cellular differentiation in the development of more complex eukaryotes.

Between 1957 and 1959, Arthur Pardee, Francois Jacob, and Jacques Monod conducted a set of experiments at the Pasteur Institute in Paris, France, that was later called the PaJaMa Experiments, a moniker derived from the researchers' last names. In these experiments, they described how genes of a species of single-celled bacteria, called Escherichia coli (E. coli), controlled the processes by which enzymes were produced in those bacteria. In 1959, the researchers published their results in a paper titled 'The Genetic Control and Cytoplasmic Expression of 'Inducibility' in the Synthesis of b-galactosidase by E. coli'. When they compared mutated strains of E. coli to a normal strain, Pardee, Jacob, and Monod identified the abnormal regulation processes and enzymes produced by the mutated genes. The results showed how enzymes break down the molecules that the bacteria ingested. The PaJaMas experiments uncovered some of the molecular mechanisms that regulate how some genes yield enzymes in many species.