"npas2" is expressed everywhere in the periphery of the body. special focus has been given to its function in liver tissues, and its mrna is upregulated in "clock"-mutant mice. however, studies have shown that "npas2" alone is unable to maintain circadian rhythms in peripheral tissues in the absence of clock protein, unlike in the scn. one theory to explain this observation is that neurons in the brain are characterized by intercellular coupling and can thus respond to deficiencies in key clock proteins in nearby neurons to maintain rhythms. in peripheral tissues such as the liver and lung, however, the lack of intercellular coupling does not allow for this compensatory mechanism to occur. a second theory as to why npas2 can maintain rhythms in clock-deficient scns but not in clock-deficient peripheral tissues, is that there exists an additional unknown factor in the scn that is not present in peripheral tissues.