FGFR3 and Severe achondroplasia - developmental delay - acanthosis nigricans: Gain-of-function mutations of the FGFR3 gene result in ACH (Figure 2), as well as related chondrodysplasias including thanatophoric dysplasia (TD) and severe achondroplasia with developmental delay and acanthosis nigricans (SADDAN) [20].