A cell is a vibrating bag of molecules, densely packed with DNA, proteins, RNAs, and lipids. The ratios of these molecules are not balanced, though. A typical HeLa cell, widely used as a model to study cancer in the laboratory, has about 20 times more protein than DNA by mass.Such imbalances are pervasive across the tree of life, but proteins are always the heaviest and most diverse group of molecules within a cell. A single human cell encodes more than 20,000 proteins, each built from 20 standa...