Achondroplasia: The most common form of short stature with disproportionately short limbs -- dwarfism with short arms and legs. Achondroplasia is caused by mutation in the fibroblast growth factor receptor-3 gene (FGFR3), which is located on chromosome 4 in chromosome band 4p16.3.

Aside from short arms and legs, the fingers are short and the ring and middle fingers diverge giving the hand a trident (three- pronged) appearance. There is a typically large head with prominence of the forehead (frontal bossing), underdevelopment (hypoplasia) of the midface with cheekbones that lack prominence, and a low nasal bridge with narrow nasal passages. The brain is normal and intelligence is entirely normal in achondroplasia. However, the complications of achondroplasia can impinge on the brain and the spinal cord.

Achondroplasia is inherited as an autosomal dominant trait affecting boys and girls equally. The parents of children with achondroplasia are more often than not normal. Most cases of achondroplasia are due to new mutations that appear for the first time in the affected children.

Achondroplasia can be diagnosed before birth by molecular means. The limited number of changes in the FGF3 gene responsible for achondroplasia and the ease with which they can be detected provide the basis for a simple method for prenatal diagnosis.