The present invention provides novel clock proteins BMAL2 (Brain-Muscle-Arnt-Like protein2), which is crucial for the clock oscillation mechanism including photic-input pathway and output pathway, novel clock genes encoding the proteins, a screening method using the proteins to screen a promoter or a suppressor of the promoter transactivation, and the like.Genes for cCLOCK, cPER2, cBMAL1 were isolated from the chicken pineal gland which is a material suitable for studying circadian clock, then cDNA encoding the novel clock protein cBMAL2 having homology with cBMAL1 was isolated and sequenced. Further, BMAL2 cDNAs in human, mouse and rat were isolated respectively from the human embryonic kidney cell line, the mouse mid brain and the rat early fibroblast, and sequences of these cDNAs were determined. BMAL2 forms a heterodimer with CLOCK or BMAL1, etc. and it also forms a homodimer.