Temporal utilization of site-specific transcription factors. (a) Percentage of the TF repertoire used during embryonic development. Samples were taken at various degrees of granularity (red, BDGP in situ database; blue, embryonic microarray gene expression time-course; green, active transcription map). Although there are differences in the absolute number of transcribed TFs, there is good agreement in the general trends. About half of the TFs are maternally contributed. The peak of TF expression is around 10 to 12 h AEL, and the number of expressed TFs declines towards the end of embryogenesis. (b) Proportion of TFs in the group of expressed genes. The ratio of TFs versus non-TFs is highest between 2 and 6 h AEL. This coincides with the functional compartmentalization of the germ layers leading to the development of various organ systems.