Recent discoveries indicate that a class of RNA molecules called small RNAs operate many of the cell’s controls. They can shut down genes or alter their levels of expression.

In some species, truncated RNA molecules literally shape genomes, carving out chunks to keep and discarding others. There are even hints that certain small RNAs might help chart a cell’s destiny by directing genes to turn on or off during development, which could have profound implications for coaxing cells to form one type of tissue or another.