FGFR3 and thanatophoric dysplasia: The most common form of dwarfism, achondroplasia (ACH), is caused by a gain-of-function mutation in the FGFR3 gene encoding fibroblast growth factor receptor 3.1 FGFR3 gain-of-function mutations are also associated with hypochondroplasia, mild dwarfism, thanatophoric dysplasia (TD), and severe and lethal dwarfism.2