Embryonic development results in animals whose body plans exhibit a variety of symmetry types. While significant progress has been made in understanding the molecular events underlying the early specification of the antero-posterior and dorso-ventral axes, little information has been available regarding the basis for left-right (LR) differences in animal morphogenesis. Recently however, important advances have been made in uncovering the molecular mechanisms responsible for LR patterning. A number of genes (including well-known signaling molecules such as Sonic hedgehog and activin) are asymmetrically expressed in early chick embryos, well before the appearance of morphological asymmetries. One of these, nodal, is asymmetrically expressed in frogs and mice as well, and its expression is altered in mouse mutants exhibiting defects in laterality. In the chick, these genes regulate each other in a sequential cascade, which independently determines the situs of the heart and other organs.