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 (Fig. 1a) was delivered to the photoreceptor layer by subretinal injections (see Methods). Transcriptional regulatory elements were chosen to direct expression preferentially in M cones, but not short- (S) wavelength-sensitive cones or rods3. To provide the receptoral basis for trichromacy, animals received three 100-ml injections (containing a total of 2.7 3 1013 viral particles) in each eye, which produced a relatively uniform, third submosaic of approximately 15–36% of M cones that coexpressed the transgene (Fig. 1e, f).