Ataxia-telangiectasia (A-T) is a progressive genetic disorder affecting the central nervous and immune systems, and involving chromosomal instability, cancer predisposition, radiation sensitivity and cell cycle abnormalities. Studies of the cellular phenotype of A-T have pointed to a defect in a putative system that processes a specific type of DNA damage and initiates a signal transduction pathway controlling replication and repair. A-T is genetically heterogeneous, with 4 complementation groups. While functional cloning of the A-T gene(s) using gene transfer has proven problematic, positional cloning attempts are zeroing in on a defined interval on chromosome 11q22-23 that probably harbors the mutations for all 4 complementation groups.