There are now 5 distinct FGFR3-related chondrodysplasias caused by specific gain-of-function mutations in FGFR3 within this family, which range from the very severe and typically perinatal lethal forms of dwarfism (thanatophoric dysplasia type I and type II) to the most common ACH and severe ACH with acanthosis nigricans and developmental delay (SADDAN) and the milder short stature condition, hypochondroplasia (HCH) (6, 10). This evidence concerns the gene FGFR3 and achondroplasia.