Expression of tailless is activated by torso protein in the poles of the embryo. Tailless is also regulated in a complex manner by the maternal-effect gene bicoid. Both embryonically transcribed hunchback and maternally transcribed hunchback are activated by bicoid protein in the anterior and is inhibited in the posterior by nanos protein. Embryonically transcribed hunchback protein is able to exhibit the same effects on
Krüppel and knirps as maternally transcribed hunchback. The Krüppel gene is activated when the bicoid protein gradient declines steeply, at the central part of the embryo. Krüppel is regulated by five regulatory proteins: bicoid, hunchback, tailless, knirps and giant. Krüppel is inhibited by high levels of hunchback, high levels of giant, and tailless, which establishes the anterior boundary of Krüppel expression. Krüppel is also inhibited by knirps and activated by low levels of bicoid and low levels of hunchback, which establishes the posterior boundary of Krüppel expression. The knirps gene appears to be spontaneously activated. It is repressed by hunchback. Hunchback repression thus defines the anterior boundary of the knirps gene. Due to more efficient inhibition of the knirps gene by hunchback, knirps is expressed more posteriorly in the embryo compared to Krüppel. Tailless protein inhibits knirps gene expression in the posterior part of the embryo, allowing the knirps protein to be expressed only in the central part of the embryo (but more posterior compared to Krüppel). This is due to the ability of both hunchback and tailless to bind to the enhancer regions of knirps. ==Mechanism of action==