Autosomal recessive syndrome of chromosomal instability Louis-Bar (ataxia teleangiectasia) was clinically diagnosed already at the age of two years of the boy. For genetic prevention in pregnancy of the mother was necessary to identify the mutation in ATM gene on 11q23. Because of high molecular heterogeneity in ATM gene the chance to detect the mutation is generally only in half of patients. The close consanguinity of the parents with F = 1/32 allows to find homozygous frameshift mutation 4143insT in exon 29. The foetus was found to be heterozygote for this mutation and the pregnancy could continued. The truncating mutation explains also the early and severe manifestation of the disorder in the boy.

        Key words: Louis-Bar syndrome (ataxia teleangiectasia), chromosomal instability, autosomal recessive inheritance, parental consanguinity, ATM gene in region 11q23, genetic prevention