Among the clock genes involved in the control system of circadian rhythms, PER1 gene aroused our interest, because its rhythmic expression was disrupted in MDD subjects (19, 30, 32), as well as polymorphisms of rs7221412 was associated with the timing of human behavioral rhythms (mean activity timing was delayed by 67 min in rs7221412GG versus rs7221412AA homozygotes) and time of death (58). Here, CLOCK is linked to major depressive disorder.