Have you ever been stuck in the middle of a crowd? As people pack closer together, it becomes more difficult to move through the crowd. Sometimes it can become so tightly packed that you cannot move at all. If this sounds uncomfortable, then you probably wouldn’t like to live inside a cell, which is densely packed with proteins and other molecules. This crowding is very important for the cell—it pushes the molecules together so that they can interact and perform the chemical reactions that the cell needs to live. In fact, many human diseases are likely influenced by changes in molecular crowding that cause harmful interactions between proteins. Despite its importance, it remains a mystery how the crowding inside cells is controlled. Combining biophysics, cell biology, physical modeling, and cryo-electron tomography, an international team of scientists at New York University (NYU) and the Max Plank Institute of Biochemistry (MPIB) has discovered that the mTORC1 signaling pathway controls the concentration of ribosomes inside the cell, thereby regulating crowding and the ability of proteins to interact with each other to form phase-separated compartments. This study is published in the journal Cell.