Fig. 1. Forced expression of secreted versions of FGFR1 and 2 blocks feather development. (A) The ligand binding regions of FGFR1-IIIc and FGFR2-IIIb (red) were fused to the Fc fragment of a mouse immunoglobulin (blue) to generated secreted receptor versions. Tm, transmembrane domain; TK, tyrosine kinase domains. (B-D) Feather phenotypes in embryos injected with RCAS-AP (B), RCAS-FGFR1-Fc (C,D,F), or RCAS-FGFR2-Fc (E). Embryos in B, C and F were injected with the retrovirus at HH9. The embryos in D and E were in injected into subectodermal mesenchyme of the back at HH18. All embryos were allowed to develop until HH38. The arrowhead in F indicates fusions between adjacent feather primordia.