The functions of the breast cancer susceptibility genes BRCA1 and BRCA2 are not fully elucidated, but appear to include the regulation of X chromosome activity. Xist is a non-coding RNA that accumulates on the inactive X chromosome and is required for X chromosome inactivation during the silencing step.¹ The RING domain of BRCA1 and Xist interact in mammalian cells and it has been suggested that BRCA1 contributes to the initiation of X chromosome inactivation.² Women with ovarian cancer possessing germline mutations in BRCA1 have been found to frequently demonstrate non-random X chromosome inactivation.³ In the light of these findings, a recent report by de la Hoya et al is of interest.⁴ In this study of 68 Spanish breast/ovarian pedigrees they reported that 67% of the children of women who carried a BRCA1 mutation were female. By contrast, only 54% of the offspring of BRCA2 carriers and 52% of the offspring of non-carriers were female. This highly skewed sex ratio in the offspring of BRCA1 carriers from Spain prompted us to ask whether this is true in other populations as well.

To address this question, we examined the sex ratios of the offspring of a total of 1040 women (229 BRCA1 carriers, 104 BRCA2 carriers, and 707 non-carriers) from five different studies which were conducted in …