Retinoic acid coordinates somitogenesis and left–right patterning in vertebrate embryos

Though symmetrical from the outside, the body plan of vertebrates and other animals is far from symmetrical inside. By the time the human heart and lung develop in the embryo they are directed to the left and right of the body cavity. Much research has gone into establishing the genetics and signalling mechanisms that impose this asymmetry. But there is a catch: some tissues, chiefly the muscles and skeleton, must ignore or overrule the instruction if they are not to become asymmetric. Three papers in this issue, and a News and Views piece by Eran Hornstein and Clifford J. Tabin, address the fascinating question of how the somites, embryonic elements that give rise to symmetrical tissues, pull off that trick.