Hereditary hyperekplexia (HH) due to deficiency of the beta subunit of the glycine receptor is a disorder of the inhibitory glycinergic neurotransmitter system. It is characterized by an exaggerated startle response to sudden external stimuli. Affected infants react with violent jerking to noise and touch, and exhibit sustained stiffening of the trunk and limbs, fist clenching, and trembling that may resemble seizures. This condition may be fatal in some cases, where the exaggerated startle response may lead to prolonged breath holding. The children display delayed achievement of motor milestones, although intellectual development is not affected.