How Drosophila Reacts to circadian rhythm? This animation shows negative feedback loop. per (period) gene is expressed, and PER proteins synthesis. pink PER proteins are degraded quickly, but as they become red they are long lasting. PER protein concentration is highest in the middle of the night, when they start entering nucleus. As they enter nucleus, they act as suppressors for per gene. As sun rises, PER proteins degrade and per gene expression start again.