The light-dark cycle of the environment serves as one of the major Zeitgebers in entrainment of the circadian clock. The circadian system consists of interconnected feedback loops in which the CCA1/LHY-TOC1 loop has a central position. Genetic analyses of the elf4 mutant suggested that it is a positive regulator of CCA1 and LHY expression. Recently, we refined the mode-of-action of ELF4 in entrainment of the clock, and here hypothesize that ELF4 expression is interlocked with the CCA1/LHY-TOC1 loop.