Figure 2.

Repressor cooperativity establishes a middle gradient zone in which distinct gene expression boundaries are possible. (A) Hh gradient and the corresponding Gli OARG. [A] and [R] range from 0 to 100 in arbitrary units, with fraction activator and repressor shown on the scale. Dashed lines indicate the boundaries of the middle gradient zone, as defined by Equations (6) and (7). (B) Under a cooperative model (ω=30), different enhancer affinities define distinct boundaries of gene activation. As enhancer affinity increases (as K increases), the region of gene activation is more restricted. (C) Under a non-cooperative model there is no middle zone in which multiple gene expression boundaries are possible, and all genes are activated when [A]>[R], regardless of affinity.