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 (per^S), lengthen (per^L), and even abolish (per⁰) the period of the circadian rhythm.