Gap gene

[1] They used a genetic screen to identify genes required for embryonic development in the fruit fly Drosophila melanogaster.

They found three genes – knirps, Krüppel and hunchback – where mutations caused deletion of particular stretches of segments.

Later work identified more gap genes in the Drosophila early embryo – giant, huckebein and tailless.

[2][3] Further gap genes including orthodenticle and buttonhead are required for the development of the Drosophila head.

The Krüppel gene is activated when the bicoid protein gradient declines steeply, at the central part of the embryo.