the npas2 protein is a member of the basic helix-loop-helix (bhlh)-pas transcription factor family and is expressed in the scn. npas2 is a pas domain-containing protein, which binds other proteins via their own protein-protein (pas) binding domains. like its paralogue, clock (another pas domain-containing protein), the npas2 protein can dimerize with the bmal1 protein and engage in a transcription/translation negative feedback loop (ttfl) to activate transcription of the mammalian "per" and "cry" core clock genes. npas2 has been shown to form a heterodimer with bmal1 in both the brain and in cell lines, suggesting its similarity in function to the clock protein in this ttfl.