Achondroplasia, the most common form of dwarfism in man, is a dominant genetic disorder caused by a point mutation (G380R) in the transmembrane region of fibroblast growth factor receptor 3 (FGFR3). We used gene targeting to introduce the human achondroplasia mutation into the murine FGFR3 gene. Heterozygotes for this point mutation that carried the neo cassette were normal whereas neo ؉ homozygotes had a phenotype similar to FGFR3-deficient mice, exhibiting bone overgrowth. This was because of interference with mRNA processing in the presence of the neo cassette. Removal of the neo selection marker by Cre͞loxP recombination yielded a dominant dwarf phenotype. These mice are distinguished by their small size, shortened craniofacial area, hypoplasia of the midface with protruding incisors, distorted brain case with anteriorly shifted foramen magnum, kyphosis, and narrowed and distorted growth plates in the long bones, vertebrae, and ribs. These experiments demonstrate that achondroplasia results from a gain-of-FGFR3-function leading to inhibition of chondrocyte proliferation. These achondroplastic dwarf mice represent a reliable and useful model for developing drugs for potential treatment of the human disease.Four fibroblast growth factor receptors (FGFRs) are known (1), and Ͼ50 mutations in three of them (FGFR1, 2, and 3) recently have been implicated in congenital skeletal and cranial disorders (reviewed in refs. 2-4). Achondroplasia, the most common form of dwarfism, was shown to be linked to a single point mutation, G380R, in the transmembrane region of FGFR3 (5, 6). FGFR3 is expressed mainly by developing bones, brain, lung, and spinal cord (7,8), and FGFR3-deficient mice show enhanced endochondral bone growth, expansion of their growth plate, and increased chondrocyte proliferation (9, 10). Thus, FGFR3 is a negative regulator of bone growth. Several experiments at the cellular level indicated that the Ach mutation (G380R) results in a constitutive activation of the receptor in a ligand-independent manner (11-13). It was suggested that this is because of stabilization of receptor dimers, a prerequisite for signal transduction in these receptors (14). This is also consistent with the constitutive activation by dimer formation described for an erbB2 (neu) receptor mutant, which carries a Val-to-Glu mutation in an analogous position to that of the FGFR3 variant in its transmembrane region (15). It is likely that, in many of the other mutations in FGFR1-3, the underlying mechanism of receptor activation is also through stabilization of receptor dimers because many of these mutations result in unpaired cysteines that may enhance inter-receptor disulfide bonds (2-4).To generate an animal model for this type of mutation and to study the role of the mutated FGFR3 in vivo, we used gene targeting to introduce the achondroplasia mutation (G380R) into murine FGFR3. This resulted in a dominant dwarf phenotype that exhibits many of the features of human achondroplasia. This is an indication from in vivo data for ...