FGFR3 and achondroplasia: Ach is an autosomal dominant disorder caused by a mutation in the short arm of chromosome 4 that affects the fibroblast growth factor receptor 3 (FGFR3) gene.1, 5 Different mutations of FGFR3 can cause hypochondroplasia, Ach, severe Ach with developmental delay and acanthosis nigricans, thanatophoric dysplasia, and many other diseases that manifest as short stature.3