sentences:  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.NPAS2-deficient mice have been shown to have long-term memory deficits, suggesting that the protein may play a key role in the acquisition of such memories.This theory was tested by inserting a reporter gene (lacZ) that resulted in the production of an NPAS2 protein lacking the bHLH domain.