Every cell in a multicellular organism has the same DNA; cells in different organs acquire their different forms and functions by using different subsets of that DNA. It’s largely a mystery how an organism conveys to its constituent cells where they are in the body, and therefore which of their genes should be active, but some pieces of the puzzle are known.
For example, in developing fruit-fly embryos, a network of so-called gap genes is involved in telling cells where they’re positioned between the head and the tail. Each of the four main gap genes, first described1 in 1980, gets its name from the consequences for a fly in which the gene is mutated: giant (Gt), hunchback (Hb), knirps (Kni, German for little person), and Krüppel (Kr, German for cripple). At a brief, specific point in development, when the embryo consists of some 6000 cells, the proteins expressed by...
