WHEN the DNA sequence of the human genome was revealed in 2000, many people expected it to start a revolution. Researchers would be able to discover the genes that caused or influenced diseases. And drug companies would be able to use that knowledge to create better medicines. Until recently, though, it has been a case of “revolution postponed”. The flood of promised discoveries has been more like a trickle. Much of the reason for the unfulfilled promises was naivity about how straightforward the link between different versions of genes and particular diseases would be. But that naivity has gone, and the fact that complex illnesses often have contributions from large numbers of genes is now recognised.