What is the main cause of breast cancer?

The causes of breast cancer aren't fully understood, making it difficult to say why one woman may develop breast cancer and another may not.

However, there are risk factors that are known to affect your likelihood of developing breast cancer. Some of these you can't do anything about, but there are some you can change:
Age
Family history
Previous diagnosis of breast cancer
Previous benign breast lump
Breast density
Exposure to oestrogen
Being overweight or obese
Being tall
Alcohol
Radiation
Hormone replacement therapy (HRT)
Contraceptive pill