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Cover The unique midkine (mdk) gene in higher vertebrates encodes a heparin-binding growth factor with neurotrophic activity that shows widespread expression duringembryonic development. In the teleost fish lineage, a large duplication resulted in the presence of two mdk genes. Functional analyses in zebrafish have now shown that duplicated mdkcopies display highly divergent expression patterns and distinct biological activities in the developingnervous system. In this figure, the expression of mdka in the central neural tube and somites (right) and mdkb in the dorsal neural tube (left) are shown together with their completely different effects on the development of floorplate and neural crest cells, respectively, after overexpression. (For details, see Winkler et al., pp. 1067–1081.)