Chromosome 17q-linkage seems to be infrequent in Icelandic families at risk of breast cancer.

Eight Icelandic families with multiple cases of breast cancer, and 17 pairs of sisters diagnosed by the age of 50 were analysed for linkage to markers around BRCA1 on chromosome 17q. The sister-pairs are thought to represent a wider population as compared to the larger high-risk families. Tumours were also analysed for LOH involving BRCA1. In accordance with a proposed tumour-suppressive function of BRCA1, and high prevalence of LOH in 'linked' tumours, the paired sisters' tumours were assayed for double LOH events with common alleles retained. No such pair was observed, and LOH events were seemingly randomly distributed at a 38% frequency. This indicates that most or all pairs are due to other genes than BRCA1 or sporadic involvement. Of the eight high-risk families, only one showed convincing evidence of 17q-linkage. Therefore, BRCA1 mutations seem to be a minor explanation of familial risk of breast cancer in Iceland.

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