To elucidate the mechanism of the circadian rhythm, genes differentially expressed during subjective day and night in the rat suprachiasmatic nucleus (SCN), a circadian oscillator in mammals, were surveyed by a differential display method. We isolated a novel gene, scop (SCN circadian oscillatory protein), that was expressed in a circadian manner in the SCN. SCOP protein is predominantly expressed in the brain and has domains including a pleckstrin homology domain, leucine-rich repeats, a protein phosphatase 2C-like domain and a glutamine-rich region. The structural feature of SCOP protein suggests its role in the intracellular signaling in the SCN.