Vertebrate left and right: finally a cascade, but first a flow?

Vertebrate development gives rise to systematic, normally reliably coordina ted left-right asymmetries of body structure. This "handed asymmetry" of an atomy must take its ultimate origin from some chiral molecular assembly (on e exhibiting no planes of symmetry and thus, having an intrinsic "handednes s") within the early embryo's cells. But which molecules are involved, how is their chiral property coordinately aligned among many cells, and how doe s it "seed" the differential cascades of gene expression that characterise right and left halves of the embryo? Recent molecular characterisations of mouse mutations that randomise or reverse body asymmetries have offered tan talising clues to the chiral initialer molecules, but the findings in a sub sequent Cell paper (Nonaka S, Yosuke T, Okada Y, Takeda S, Harada K, Kanai Y, Kido M, Hirokawa N. Randomisation of left-right asymmetry due to loss of nodal cilia generating a leftward flow of extraembryonic fluid in mice lac king KIF3B motor protein. Cell 1998;95:829-837. [Reference 1 ]) may help us understand how the first gene expression asymmetries occur. (C) 1999 John Wiley & Sons, Inc.