 The effects of these genes interact; for instance, the Bicoid protein blocks the translation of "caudal"'s messenger RNA, so the Caudal protein concentration becomes low at the anterior end.Caudal later switches on genes which create the fly's hindmost segments, but only at the posterior end where it is most concentrated.The Bicoid, Hunchback and Caudal proteins in turn regulate the transcription of gap genes such as "giant", "knirps", "Krüppel", and "tailless" in a striped pattern, creating the first level of structures that will become segments.