The oscillations of circadian genes control the daily circadian clock, regulating a diverse array of physiologies with the 24-hour light/dark cue across a wide variety of organisms. Here we first show that before embryonic circadian rhythms occur, the oscillation (nucleocytoplasmic shuttling) of core circadian gene Clock is tissue-specific and correlated with the state of differentiation during both early development and later pancreas organogenesis. Disruption of Clock as well as Timeless in the embryonic pancreas does not block pancreatic differentiation but alters the balance and maturity of endocrine and exocrine cells. Molecular analysis indicates that inhibition of Clock or Timeless expression disturbs not only cell cycle regulators, but also Wnt- and Notch-signaling components, whose oscillations establish the timing mechanism in somitogenesis. Thus, our results provide new insights about circadian genes' function in control of the timing of differentiation during embryonic development. (c) 2007 Elsevier Inc. All rights reserved.