An 8-year-old girl with severe microcephaly of prenatal onset, borderline intelligence, defects of skin pigmentation, deficiency of both humoral and cellular immunity, a normal serum alpha-fetoprotein level and hypersensitivity to ionizing irradiation is described. Spontaneous chromosomal breakage in lymphocytes together with the clinical presentation led to the diagnosis of ataxia telangiectasia variant (AT-V). In addition, the patient carried a constitutional translocation of paternal origin: 46,XX,t(3;7)(q12;q31.3) pat. In subsequent linkage and haplotype studies in 12 AT-V families with microsatellite markers from each of the translocation breakpoint regions, we could clearly exclude the localization of an AT-V gene to these regions.