Gap genes were first described by Christiane Nüsslein-Volhard and Eric Wieschaus in 1980. 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". Further gap genes including orthodenticle and buttonhead are required for the development of the "Drosophila" head.