One of the puzzling features of the human genome is that although genes are numerous they actually form less than 5% of the DNA in a cell nucleus. The rest was thus, rather cavalierly, dubbed “junk DNA” by those who discovered it. Gradually, a role for some of this junk has emerged. In particular, parts of it regulate the activity of genes, and thus which proteins are produced and in what quantities. That has implications for what a cell does—or, to put it another way, what type of cell it is.