In a living cell, gene expression—the transcription of DNA to messenger RNA followed by translation to protein—occurs stochastically, as a consequence of the low copy number of DNA and mRNA molecules involved. Can one monitor these processes in a living cell in real time?  How do cells with identical genes exhibit different phenotypes?  Recent advances in single-molecule imaging in living bacterial cells allow these questions to be answered at the molecular level in a quantitative manner. It was found that rare events of single molecules can have important biological consequences.