In this report, the frequency of the G-->A transition polymorphism at nucleotide 5557 in exon 39 of the coding sequence of the gene mutated in ataxia-telangiectasia (ATM) was analysed. The frequency of the A and G alleles was estimated in the general population at 0.15 and 0.85, respectively. This polymorphism can be identified by single-strand conformation polymorphism (SSCP) and by a simple and rapid Ddel digestion after polymerase chain reaction (PCR)-mediated site directed mutagenesis (PSDM). (C) 1999 Academic Press.