Why gene therapies don't work for everyone

Growing up in Devon in the 1980s, brothers James* and Matt* had to be careful with their antics–a cut or bruise could land them in hospital.

They have haemophilia, a genetic disease where the blood doesn’t clot efficiently due to a lack of a clotting protein. It’s plagued their family for generations. Their granny’s brother died from it, and they, their uncle and a younger cousin have long managed it by injecting the missing protein to prevent bleeds.

While haemophilia can now be managed effectively–in high-income countries at least, where medication is readily available–it can severely limit freedom and quality of life and can even reduce life expectancy. Bleeds can be triggered by minor strains that healthy people barely notice, such as from walking or lifting boxes. Blood then seeps from the capillaries and pools in the joints, causing throbbing pain and lasting damage.