PER2, a mammalian homologue of the Drosophila period gene, shares a 40% homology with PER1 including the protein dimerization PAS domain. PER2 is a circadian regulator that may act as a transcription factor. It behaves as a negative element in circadian transcriptional loop. PER2 does not appear to bind DNA, suggesting indirect transcriptional inhibition. Expression oscillates with a 24 hour rhythm in the suprachiasmatic nucleus (SCN) and the whole eyes. Oscillations are maintained under constant darkness and are responsive to changes of the light/dark cycles. There is a 4 hour time delay between PER1 and PER2 oscillations. The expression rhythms appear to originate from retina.