The Economist explains: Why gene therapy just got easier

SOME diseases, such as haemophilia and cystic fibrosis, are caused by broken genes. Doctors have long dreamed of treating them by adding working copies of these genes to cells in the relevant tissue (bone marrow and the epithelium of the lung respectively, in these two cases). This has proved hard. There have been a handful of qualified successes over the years, most recently involving attempts to restore vision to people with gene-related blindness. But this sort of gene therapy is likely to remain experimental and bespoke for a long time, as it is hard to get enough genes into enough cells in solid tissue to have a meaningful effect.Recently, though, new approaches have been devised. Some involve editing cells’ genes rather than trying to substitute them. Others create and insert novel genes—ones that do not exist in nature—and stick those into patients. Both of these techniques are being applied to cells from the immune system, which need merely to be injected into a patient’s bloodstream to work. They therefore look susceptible to being scaled up in a way that, say, inserting genes into retinal cells is not.Gene editing can be done in at least two ways. One, CRISPR-Cas-9 editing, employs modified versions of a natural antiviral defence found in bacteria, which recognises and cuts specific sequences of DNA bases (the “letters” of the genetic code). The other, ...