Expression profiles of six yeast genes across multiple cell-cycle microarray time courses. Expression profiles for (a) budding yeast CDC5 and DBF2, and (b) their fission yeast orthologs plo1 and sid2. These four genes are all periodically expressed according to our analysis [13,14] but not according to that of Lu and co-workers . (c) Conversely, fission yeast mcm3 and mcm5 are both periodically expressed according to the analysis of Lu et al.  but not according to us [13,14,17]. The information in the panels refers to the experiments from which the data come and the method of cell-cycle arrest; for example 'Cho et al.  CDC28' indicates a time-course experiment in which the cells were arrested using a CDC28 mutant. The values on the y-axis on each profile indicate the log2 ratio between the expression at a given time point compared with the average expression across the profile. The rank scores show that plo1 and sid2 are both among the top 100 cycling genes according to our analysis, whereas mcm3 and mcm5 are among the 3,000 least cycling genes. All plots were obtained from the Cyclebase.org database where further details on the normalization procedure and the scoring scheme can also be found [17,38].