In human cells, there are about 20,000 genes on a two-meter DNA strand—finely coiled up in a nucleus about 10 micrometers in diameter. By comparison, this corresponds to a 40-kilometer thread packed into a soccer ball. Despite this cramped space, stem cells manage to find and activate the correct genes in a matter of minutes. Which genes these are differs from cell to cell. Precise activation is crucial as errors in gene selection can lead to disease or cell death.