Researchers have spent years taking apart one of the world’s simplest microbes, Mycoplasma pneumoniae, piece by piece, and created a detailed list of what molecular parts the living cell can and cannot do without, knowledge that could accelerate the development of “living medicines” built from this very microbe. Their efforts have revealed how much real estate engineers have to edit and repurpose the bacterium for therapeutic purposes, for example to combat antibiotic resistance or cancer. The study, published today in Molecular Systems Biology, is the most comprehensive “essentiality map” for any living organism built to date.