Fig 1. The Bicoid system transforms the gradual information contained in the Bicoid concentration gradient into a step-like response in 30 min.

A) At nc 8 (T = 1 hr), the Bicoid exponential gradient (red) is steadily established [5, 6] with its highest concentration at the anterior pole (A) and its lowest concentration at the posterior pole (P). The first hints of zygotic transcription are detected by RNA FISH marking the onset of zygotic transcription [15]. B) At nc 11 (T = 1 hr 30 min), the main Bicoid target gene, hunchback (hb), is expressed within a large anterior expression domain. hb expression is schematized here from RNA FISH data [15]: nuclei where ongoing transcription at the hb loci is detected are shown in green and nuclei silent for hb are shown in white.