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. Mutations in the per gene can shorten (
perS), lengthen (
perL), and even abolish (
per0) the period of the circadian rhythm.