Gene therapy was performed on adult squirrel monkeys (Saimiri sciureus) that were missing the L-opsin gene. In this species, some females have trichromatic colour vision whereas males are red–green colour blind2. Serotype 2/5 recombinant adeno-associated virus (rAAV) containing a human L-opsin gene under the control of the L/M-opsin enhancer and promoter