Diagram depicting the activity of the per and tim genes in a single cell of a fruit fly. Four depictions are shown. The first diagram corresponds to early in the night. The per and tim genes are active, producing mRNA that moves to the cytoplasm where it is translated into PER and TIM proteins. The second diagram shows the PER and TIM binding to each other. The third diagram shows the complex of PER and TIM proteins moving into the nucleus and binding to the per and tim genes, stopping their transcription. The last diagram shows the PER/TIM protein complex breaking down, allowing the per and tim genes to be transcribed, thereby starting another cycle.