predicted human per2 protein created from the "per2" gene shares 44% identity with human "per1" and 77% identity with mouse "per2". northern blots analysis revealed that "per2" was expressed as a 7-kb mrna in all tissues examined. an additional 1.8-kb transcript was also detected in some tissues. "per2" mrna has been shown to peak at zt 6 in the scn.