TheClock (Clk)mutation shortens circadian rhythms of locomotor activity and eclosion from ca. 24 h to 22.5–23 h.Clkwas previously mapped, by meiotic recombination, very close to theperiod(per)locus on theXchromosome. To determine whetherClkis a mutation within thepergene or if the former is separate from the latter, two overlapping genomic fragments were cloned fromClkflies to produce aper-containing 13.2 kb construct,per01flies (which by themselves are arrhythmic)—when transformed with this construct—-expressed short-period rhythms. This indicates that theClkmutation is contained within this 13.2 kb region and is almost certainly a new“fast-clock”allele ofper.