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Retinitis pigmentosa 1 (RP1) is a common inherited retinopathy with variable onset and severity. TheRP1gene encodes a photoreceptor-specific, microtubule-associated ciliary protein containing the doublecortin (DCX) domain. Here we show that another photoreceptor-specific Rp1-like protein (Rp1L1) in mice is also localized to the axoneme of outer segments (OSs) and connecting cilia in rod photoreceptors, overlapping with Rp1.Rp1L1−/− mice display scattered OS disorganization, reduced electroretinogram amplitudes, and progressive photoreceptor degeneration, less severe and slower than inRp1−/− mice. In single rods ofRp1L1−/−, photosensitivity is reduced, similar to that ofRp1−/−. While individual heterozygotes are normal, double heterozygotes ofRp1andRp1L1exhibit abnormal OS morphology and reduced single rod photosensitivity and dark currents. The electroretinogram amplitudes of double heterozygotes are more reduced than those of individual heterozygotes combined. In support, Rp1L1 interacts with Rp1 in transfected cells and in retina pull-down experiments. Interestingly, phototransduction kinetics are normal in single rods and whole retinas of individual or doubleRp1andRp1L1mutant mice. Together, Rp1 and Rp1L1 play essential and synergistic roles in affecting photosensitivity and OS morphogenesis of rod photoreceptors. Our findings suggest that mutations inRP1L1could underlie retinopathy or modify RP1 disease expression in humans.