Photoreceptors of the Xenopus laevis retina are the site of a circadian clock Ar part of a differential display screen for rhythmic gene products in this system, we have identified a photoreceptor-specific mRNA expressed in peak abundance at night, cDNA cloning revealed an open reading frame encoding a putative 355 amino acid protein that we have named ''nocturnin'' (for night-factor). This protein has strong sequence similarity to the C-terminal domain of the yeast transcription factor, CCR4, as well as a leucine zipper-like dimerization motif, Nocturnin mRNA levels exhibit a high amplitude circadian rhythm and nuclear run-on analysis indicates that it is controlled by the retinal circadian clock at the level of transcription, Our observations suggest that nocturnin may function through protein-protein interaction either as a component of the circadian clock or as a downstream effector of clock function.