1. Institutes for Medical Psychology and
2. Physiological Chemistry, Ludwig Maximilians University, Goethestrasse 31-33, 80336 München, Germany

Correspondence to: Martha Merrow¹Till Roenneberg¹ Correspondence and requests for materials should be addressed to T.R. (e-mail: Email: till.roenneberg@imp.med.uni-muenchen.de) or M.M.(e-mail: Email: martha@imp.med.uni-muenchen.de).

Circadian clocks consist of three elements: entrainment pathways (inputs), the mechanism generating the rhythmicity (oscillator), and the output pathways that control the circadian rhythms. It is difficult to assign molecular clock components to any one of these elements. Experiments show that inputs can be circadianly regulated^{1, 2, 3} and outputs can feed back on the oscillator⁴,⁵. Mathematical simulations indicate that under- or overexpression of a gene product can result in arrhythmicity, whether the protein is part of the oscillator or substantially part of a rhythmically expressed input pathway⁶. To distinguish between these two possibilities, we used traditional circadian entrainment protocols⁷,⁸ on a genetic model system, Neurospora crassa.