Editor—Mutations of theBRCA1 and BRCA2tumour suppressor genes have been identified in some cases of familial and early onset breast cancer.1 2 Mutations of these genes, however, account for a relatively small proportion of the total cases of female breast cancer. Male breast cancer is a very rare disease, accounting for approximately 1% of all cases of breast cancer. Less is known about the genetic influences in its development. Male breast cancer has been linked to mutations of theBRCA2 gene in some cases, with the frequency of mutations varying widely (from 4-40%) in those series studied.3 4

It has been suggested that there may be other genetic factors that confer a lower absolute risk to the person, but potentially could result in a substantial number of cases within a whole population.5 We have already shown that a polymorphism in the CYP17 gene is associated with an increased risk of male breast cancer.6

A region within exon 1 of the gene coding for the androgen receptor (located on chromosome Xq11-12) is highly polymorphic and contains a variable number of CAG repeats. The variability of the number of these repeats between different ethnic populations in the USA has been studied.7 In vitro studies have shown that a relatively short CAG repeat sequence increases the level of transactivation of the androgen receptor.8 The androgen receptor itself binds dihydrotestosterone and therefore is one factor in the regulation of the growth of prostate cells. This may account for the finding that short CAG repeat sequences have been associated with a higher risk of developing prostate cancer. …