 In the early embryo, the "bicoid" and "hunchback" genes are at high concentration near the anterior end, and give pattern to the future head and thorax; the "caudal" and "nanos" genes are at high concentration near the posterior end, and give pattern to the hindmost abdominal segments.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.