sentences:  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.