Breast cancers in men are often oestrogen receptor positive (ER+). In fact ER+ breast cancers are more common in men than women. About 4 out of 5 breast cancers in men contain oestrogen receptors.

In order to grow, many breast cancers rely on supplies of sex hormones, particularly oestrogen. On the surface of the cancer cells are proteins called receptors. Cancers with oestrogen receptors on the surface of their cells are 'oestrogen receptor positive' (ER+). Cancers without these receptors are 'oestrogen receptor negative' (ER-).

In women the chances of a breast cancer being ER+ increases with age. The older a woman is the more likely her breast cancer is to be ER+. But, in men the frequency of ER+ breast cancers is the same at all ages.

Knowing whether or not a breast cancer contains ER helps doctors to decide on treatment. A tumour that is ER+ is more likely to respond to hormonal treatments such as tamoxifen or anastrozole. A tumour that is ER- won't benefit from these therapies.