Gene located on the X chromosome of Drosophila melanogaster
Period (per) is a gene located on the X chromosome of Drosophila melanogaster. Oscillations in levels of both per transcript and its corresponding protein PER have a period of approximately 24 hours and together play a central role in the molecular mechanism of the Drosophila biological clock driving circadian rhythms in eclosion and locomotor activity.[1][2] Mutations in the per gene can shorten (perS), lengthen (perL), and even abolish (per0) the period of the circadian rhythm.[1]