neuronal pas domain protein 2 (npas2) also known as member of pas protein 4 (mop4) is a transcription factor protein that in humans is encoded by the "npas2" gene. npas2 is paralogous to clock, and both are key proteins involved in the maintenance of circadian rhythms in mammals. in the brain, npas2 functions as a generator and maintainer of mammalian circadian rhythms. more specifically, npas2 is an activator of transcription and translation of core clock and clock-controlled genes through its role in a negative feedback loop in the suprachiasmatic nucleus (scn), the brain region responsible for the control of circadian rhythms.