Schematic diagram of the locations of conserved tyrosine residues in FGFR3 and mutant constructs designed for this study. Full-length FGFR3 contains an N-terminal extracellular ligand-binding domain, a transmembrane domain (TM), and a split cytoplasmic tyrosine kinase (TK) domain. The activating mutation K650E in the activation loop is indicated by an asterisk, and 6 conserved tyrosine residues are marked by. The bottom panel shows the spectrum of FGFR3 mutants generated. All the mutants contain the activating mutation K650E except FGFR3 wild type. Single or multiple substitutions of the non–activation loop tyrosine residues Y577, Y724, Y760, and Y770 were introduced into the FGFR3 TDII (K650E). The FF4F mutant contains the mutations at the 2 activation loop tyrosine residues (Y647/Y648). All the numbering of mutations is as for native human FGFR3.