Required for the differentiation of photoreceptor cells. Plays a role in the organization of outer segment of rod and cone photoreceptors. Interacts with RP1; has a synergistic effect with RP1 in photoreceptor differentiation. Localized to the axoneme of outer segments and connecting cilia in rod photoreceptors. Retinal-specific; expressed in photoreceptor. Detected in retina at birth but not at prenatal stages. Mice display scattered outer segment disorganization, reduced electroretinogram amplitudes, and progressive photoreceptor degeneration. In single rods defective cells photosensitivity is reduced. Rp1 and Rp1l1 double heterozygotes exhibits abnormal outer segment morphology and reduced single rod photosensitivity and dark currents, while individual heterozygotes are normal. Truncated N-terminus.