Ataxia-telangiectasia (A-T), also called Louis-Bar syndrome, is a rare genetic disease. It affects multiple systems. Individuals with A-T usually die by their early 20s. Nervous system abnormalities become apparent by age two, and muscle control progressively diminishes. Immune system deficiencies and blood cancers are common, and affected individuals are extremely sensitive to radiation.

A-T first appeared in the medical literature in the mid-1920s, but was not named as a specific disorder until 1957. The name is a combination of two recognized disorders: ataxia (lack of muscle control) and telangiectasia (tiny, red spots). However, A-T is more than the sum of these two disorders. Other A-T- associated indicators include immune system deficiencies, extreme sensitivity to radiation, and blood cancers.

Medical researchers initially suspected that multiple genes (the units responsible for inherited features) were involved. However, in 1995, mutations in a single large gene were identified as causing A-T. The gene...