The TrkA NGF receptor extracellular region contains three leucine repeats flanked by cysteine clusters and two immunoglobulin-like domains that are required for specific ligand binding. Deletion of the immunoglobulin-like domains abolishes NGF binding and causes ligand independent activation of the receptor. Here we report a specific mutation that increases the binding affinity of the TrkA receptor for NGF. A change of proline 203 to alanine (P203A) in the linker region between the leucine repeats and the first Ig-like domain increased NGF binding by decreasing the ligand rate of dissociation. This mutated receptor was appropriately expressed on the cell surface and promoted ligand-independent neurite outgrowth in PC12nnr5 cells. The mutant receptor was capable of spontaneous dimerization and was constitutively phosphorylated in the absence of ligand. Moreover, expression of TrkA-P203A receptor in fibroblasts induced DNA synthesis and transformation and generated tumours in nude mice. These data suggest that domains outside of the immunoglobulin-like structure contribute to ligand binding and constitutive activation of Trk receptors.