mdk2 expression is independent of FGF8 and nodal-related1 signaling, inhibited by BMP signaling, and upregulated by low doses of exogenously applied RA. (A, B) Lateral views of mdk2 expression in 24-hpf wild-type (wt, A) and acerebellar (ace, B) mutant embryos. mdk2 expression at the MHB (arrow) is absent due to lack of cerebellar structure. Otherwise expression appears unaffected (arrowheads). ey, eye. (C, D) Dorsal views of mdk2 (arrowhead) and gsc (arrow) expression in wt (C) and cyclops (cyc, D) mutant embryos at 90% epiboly. gsc is repressed in cyc embryos, but mdk2 expression appears unaffected. (E–P) Lateral views, dorsal to right. (E, F) Expression of eve1 (arrowhead) and mdk2 (arrow) in wt and chordino (chd) mutant embryos at 70% epiboly. eve1 expression is expanded in chd embryos, but mdk2 is repressed (arrow in F compared to E). (G–J) Expression of eve1 (arrowhead) and mdk2 (arrow) in wt (G, I), snailhouse (snh, H), and swirl (swr, J) mutant embryos at 80% epiboly. While eve1 is repressed in snh and swr, mdk2 expression is ventrally expanded. (K–P) Expression levels of mdk2 (arrowhead) are dependent on the dose of exogenously applied RA. (K–N) Dose response of mdk2 expression in 70% epiboly embryos. RA applied at 10-7 M reduces mdk2 expression (L), whereas lower doses (M, N) increase expression when compared to untreated embryos (K). (O) Expression of mdk2 in an untreated embryo at 70% epiboly. (P) Complete repression of mdk2 expression by treatment with RA at a dose of 10-6 M.