Circadian rhythm is one of the most fascinating and complex biological phenomenon's. The circadian clock controls biological activities on daily light-dark cycles in species from cyanobacteria to humans. The circadian clock has three major components: A photoactive pigment (chromophore) for sensing light and transmitting light signals, the circadian clock that oscillates every ~24-hrs, and the genes controlled by the circadian clock to bring about the physiological and behavioral changes. Several genes (Drosophila Clock Per, Tim; mammalian Per1, Per2, Per3, MOP3 and MOP4, and BMAL1) have been linked to rhythmicity or circadian behavior of living organisms. However, very little is known about the mammalian photosensory molecules. Basic-helix-loop-helix-PAS orphan protein, MOP3 (Members Of PAS Superfamily; also known as BMAL1/JAP3/PAS3; 626 aa) is a general dimerization partner for several PAS superfamily of transcription regulators. MOP3 interacts with MOP4 (also known as NPAS2; 626 aa), Clock, HIF1a, and HIF2a. MOP4 is a brain specific homolog of Clock.