Exon 9 was replaced with one in which nucleotide substitutions result in the amino acid substitution of cysteine for tyrosine at position 367 (Y367C). This mutation was corresponds to the Y373C mutation in human patients with thanatophoric dysplasia type I and causes ligand independent activation of the receptor. A floxed neo was inserted downstream of exon 9 and removed by germ line, cre mediated recombination.