Model of the Arabidopsis clock, emphasizing the roles of the TOC1 / PRR (red) and CCA1 / LHY (blue) genes. The model is oversimplified to illustrate the interlocked feedback loops. Not all known clock components are included and undoubtedly more components remain unidentified. The central loop of CCA1, LHY, TOC1, and 'X' is shaded blue. The second loop of CCA1 and CHE is shaded green. The third ('morning') loop of CCA1, LHY, PRR9, PRR7, and PRR5 is shaded yellow, and the fourth ('evening') loop of TOC1 and 'Y' is shaded gray.