While additional decades may be required

to reach a similar state of sophistication in the analysis of mammalian clockwork function, the progress made in this field has been nevertheless extraordinary. During the past 10 years, an impressive repertoire of molecular cogwheels has been established, and we are beginning to understand how these cogwheels Inhibitors,research,lifescience,medical are intertwined. The discovery of cell-autonomous and self-sustained molecular oscillators in virtually every body cell led to a paradigm change of how the clockwork circuitry governs overt rhythms in behavior and physiology. It now appears that the mammalian timing system resembles an extensive and hierarchically structured web of cellular oscillators, whose phases must be coordinated at the single cell level by

the master pacemaker in the Inhibitors,research,lifescience,medical SCN. We are also beginning to understand how molecular clocks in individual peripheral cells cooperate with cell typespecific and inducible mechanisms to optimize metabolism and physiology. Despite these advances, an important and scientifically Inhibitors,research,lifescience,medical challenging issue remains to be addressed. Although Inhibitors,research,lifescience,medical evolution-based arguments leave little doubt as to the importance of a well-functioning circadian clock for survival under natural conditions, it has been difficult to show its contribution to fitness of mammalian organisms in the laboratory. The association of increased morbidity to clock gene mutations does not address this issue in a satisfactory fashion, since such genes may execute important functions unrelated to circadian Inhibitors,research,lifescience,medical rhythm

generation (for example control of ossification by clock genes143, 144). In cyanobacteria (Synechococcus elongatus)145, 146 and a green plant (Arabidopsis thaliana)147 the benefit of circadian timing was demonstrated by an ingenious until and convincing strategy. In both species, a clock resonating with imposed light-dark cycles has been shown to increase performance and fitness. Since, depending on the imposed environmental conditions, the same clock gene mutation can be beneficial or deleterious in such PLK inhibitor experiments, the observed phenotypes must thus be caused by a rhythm-related property of the gene mutation under study. Eventually this approach should succeed in mammals as well, given the availability of mutant mice and hamsters with aberrant period length.