 In "Drosophila", the negative regulator PER, from the PER/TIM complex, is eventually degraded by a casein kinase-mediated phosphorylation cycle, allowing fluctuations in gene expression according to environmental cues.These proteins mediate the oscillating expression of the transcription factor VRILLE (VRI), which is required for behavioral rhythmicity, "per" and "tim" expression, and accumulation of PDF (pigment-dispersing factor).