There are people with a condition called hyperekplexia, which is also known as "startle disease". The reaction of being startled is so exaggerated that the people suffering from it can often be injured or even killed when they're surprised.

It's also one cause of sudden infant death syndrome (SIDS). Now, researchers have discovered that the condition seems largely connected to one gene.

According to Live Science:
The disease is caused by multiple genetic mutations, the end result of which is a failure of nerve cells to properly communicate. Specifically, the mutations affect how a molecule called glycine is moved between cells.

Normally, glycine sends inhibitory signals, which dampen a person's responses to noise and sound. In people with startle disease, these inhibitory signals are not received, and the result is an amplified, harmful response.

In the new study, researchers at the University College London and Swansea University in Wales analyzed information from 93 patients with startle disease across the globe, and identified 19 new genetic mutations. The mutations were all in the genes that encode a protein called GlyT2, which is responsible for transporting glycine into cells.
Future therapies could target GlyT2, and help prevent anyone from ever being scared to death.