Exponential concentration profile · positional information · scaling
Bicoid is a transcription factor in Drosophila that forms a concentration gradient from anterior to posterior: C(x) = C₀·exp(−x/λ). Cells read local concentration and activate different target genes above specific thresholds, establishing the body plan. Crucially, the ratio λ/L remains ~0.2 across fly species of different sizes — the gradient scales with embryo length. Noise in the gradient limits positional precision.