The multisystem autosomal recessive disease ataxia-telangiectasia (A-T) is determined by several genes, as evidenced by the existence of four complementation groups in this disorder. Using linkage analysis, the ATA (A-T complementation group A) gene was previously localized to chromosome 11, region q22-q23. Analysis of the segregation of RFLP markers from this region in a Jewish-Moroccan family assigned to group C indicates that the ATC (A-T complementation group C) gene localizes to chromosome 11q22-q23 as well.