Only about 5-10% of breast cancers are due to inheriting abnormal genes. So most breast cancers happen to women who haven't inherited a faulty gene.

Having a faulty breast cancer gene doesn't mean you will definitely get breast cancer. But, your risk of developing breast cancer is higher than someone who doesn't have an abnormal gene and you are more likely to develop breast cancer at a younger age.

The two main genes that are known to increase the risk of breast cancer in families are BRCA 1 and BRCA 2.

Men or women can have faulty breast cancer genes. So it's possible to inherit a breast cancer gene from your mother or your father.

Men with a faulty BRCA2 gene have a much higher risk of developing breast cancer than men on average. However breast cancer in men is very rare. So, most men with BRCA don't develop breast cancer.

Having a faulty BRCA1 or BRCA2 gene can also increase the risk of other cancers. In particular, ovarian cancer in women and prostate cancer in men.

There are two other gene faults, which can lead to breast cancer in families. They are much rarer than BRCA and probably account for only 1-2% of all familial breast cancers. These are the P53 gene and the AT (ataxia telangiectasia) gene.

In families where a faulty gene has been identified, only people in the family who carry the gene mutation have an increased risk.

We all have two copies of every gene. Someone who has a faulty BRCA 1 or BRCA 2 gene has one normal and one faulty copy of the gene. We only pass one copy of each gene to our children. So, each child has a one in two chance of inheriting the fault if one of their parents has a gene mutation. This means a child might not inherit the faulty gene, even if one of their parents has it.