the rna levels of mper3 oscillate with a circadian rhythm in both the scn and in the eyes, as well as in peripheral tissues, including the liver, skeletal muscle, and testis. unlike per1 and per2, of which the mrna is induced in response to light, per3 mrna in the scn does not respond to light. this suggests that per3 may be regulated differently than either per1 or per2.