Breast cancers are also categorised by whether there is an increased level of receptors for hormones. About three quarters of breast cancers produce additional oestrogen receptors on their cell surfaces, and growth is stimulated by oestrogen binding with the receptors. Oestrogen-receptor positive (ER+) cancers can be restricted by blocking the receptors or reducing oestrogen levels.⁶

Another significant receptor is the human epidermal growth factor receptor 2 (HER2). This has a role in normal cell growth, but increased gene expression occurs in about a fifth of cancers stimulating the over-production of receptors. Cancers which are HER-2 positive (HER2+) have an increased tumour growth rate.^11,12

Fortunately, HER2+ cancers are very responsive to targeted HER2 antagonist treatments such as monoclonal antibodies (MABs) and tyrosine kinase inhibitors (TKIs), with a reasonably good prognosis.

Cancers which do not have increased levels of oestrogen, progesterone and HER2 receptors account for around 15 per cent of breast cancers and can be called triple negative cancers. Some triple negative breast cancers may, however, have a BRCA1 gene fault. Metaplastic cancers and basal type breast cancers are often triple negative.^7,13,14