Classification of left-right patterning defects in zebrafish, mice, and humans

Numerous genes and developmental. processes have been implicated in the est ablishment of the vertebrate left-right axis. Although the mechanisms that initiate left-right patterning may be distinct in different classes of vert ebrates, it is clear that the asymmetric gene expression patterns of nodal, lefty, and pitx2 in the left lateral plate mesoderm are conserved and that left-right development of the brain, heart, and gut is tightly linked to t he development of the embryonic midline. This review categorizes left-right patterning defects based on asymmetric gene expression patterns, midline p henotypes, and situs phenotypes. In so doing, we hope to provide a framewor k to assess the genetic bases of laterality defects in humans and other ver tebrates. (C) 2001 Wiley-Liss,Inc.