FGFR3 and thanatophoric dysplasia type 1: 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).