PER1 and retinal degeneration: 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).