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Fig. 4 Effect of cyclopamine on midline ptc1 expression and on fin development. Whole-mount in situ hybridizations. Anterior to the left. (A-C) Dorsal to the top, 25 hour embryos; (D-L) distal to the top: 32 hour pectoral fin buds (D-J); 48 hour pectoral fin buds (K,L). (A,D-F,L) Wild type; (B) syu; (C,G-K) wild type treated with cyclopamine. (A-C,H) ptc1 RNA; (D,G,J-L) shh RNA; (E) ehh RNA; (F) twhh RNA; (I) hoxd-11 RNA. Note the partial reduction of midline ptc1 expression in syu mutants and the complete absence in cyclopaminetreated embryos (A-C). ehh and twhh are not expressed in 32 hour pectoral fin buds (E,F). Cyclopamine-treated fin buds fail to express ptc1 (H), but activate shh and hoxd-11 (G,I). shh expression subsequently becomes weaker (J) and is lost (K). cyc, cyclopamine.