Fig. 8. Mechanism of FGFR1 function. Based on data presented here, we propose that in limb development, FGFR1 serves as a principle receptor for AER-FGF signaling. Activation of FGFR1 then impacts limb skeletal formation through three distinct cellular mechanisms. As elaborated in the Discussion, FGFR1 regulation of Shh and 5'-Hox gene expression probably contributes to the molecular mechanism underlying FGF function during limb bud development.