Proteins are the go-getters of a cell. They carry out all the jobs that are required for cells to grow, reproduce and perform other duties. But genes carry instructions for far more proteins than are present in a cell at any one time. Therefore, it’s useful for researchers to know the set of proteins present, or the proteome, in a cell or tissue at a given time and under certain conditions. Understanding the size limits of a proteome and a cell’s tolerance for protein diversity is critical for determining how organisms evolve.