Mutant flies had abnormally prolonged expression of PER and TIMELESS (another component of the Drosophila negative arm) and hallmark circadian activity disturbances (Farago et al., 2019) indicating a disrupted TTFL In mammals, PER1 and PER2 are also important players in photic entrainment., and both are deregulated in symptomatic HD mice with retinal degeneration (Morton et al., 2005). The gene discussed is PER1; the disease is Huntington disease.