Much of the success of these investigations is based on years of research on the molecular mechanisms involved in the regulation of cellular growth. Normally, there are a series of factors that control cell proliferation by either stimulating growth or causing inhibition of growth, or even programmed cell death. This was interpreted as indicating that in these families there was an inherited predisposition to the development of cancer. It is known that women who have a first degree relative with breast cancer have a substantially increased risk of developing the disease in their lifetime. In aggregate, these findings are consistent with the existence of genes that predispose an individual to develop breast or ovarian cancer.
Breast cancer is the most commonly seen malignancy in women in developed countries.
A second breast cancer susceptibility gene called BRCA2 has also been identified. Defects in this gene may account for the majority of cases of familial breast cancer that are not due to mutations in BRCA1. The risk of ovarian cancer in women carrying genetic alterations in BRCA2 appears to be lower than that seen with BRCA1 mutations. This will allow presymptomatic screening of other family members to determine which individuals are at high risk for development of breast and ovarian cancer. The existence of this large number of possible mutations implies that at the current time, screening of women in the general population for ALL alterations in the BRCA1 gene would be extremely challenging.