Around four per cent of the population – almost all of them male – have the most common form of colour blindness, which prevents them distinguishing between red and green. The principal cause is a faulty gene in the light-receiving retina. As such, a cure requires replacement of the faulty gene by properly-functioning versions in so-called ‘gene therapy’ – and some progress has been made.
In 2009, the journal Nature carried a report by a team of researchers in the US in which full colour vision had been restored to two squirrel monkeys. The feat was achieved by using a deactivated virus to carry fresh copies of the gene into the eye cells of the monkeys. Tests are continuing to find out if the treatment could work with humans.