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The PER2 gene at 2q37.3 codes for the 1255 amino acid period circadian protein homolog 2. there are two isoforms. The processed homodimer is a transcriptional repressor which is critical for circadian rhythm. This oscillates diurnally in both central and peripheral clocks. Targeted degradation of mPER and mCRY proteins enables the reactivation of the CLOCK and ARNTL genes.

Familial advanced sleep phase syndrome 1 (FASPS1, OMIM:604348) is caused by the variant 662S → G and is characterised by very early sleep onset and offset (morning larks).