Breast cancer cell, coloured scanning electron micrograph (SEM). Breast cancer forms when abnormal cells divide rapidly in an uncontrolled and chaotic manner and clump together to form tumours. Breast cancer is the most common form of cancer in women with 1 in 8 women in the UK developing breast cancer during their lifetime. 1 in 810 men will also develop breast cancer at some point in their lifetime. 78% of people with breast cancer survive 10 years or more and, as with many cancers, early diagnosis can assist with a good prognosis. Magnification: x4000 when printed at 10 cm wide.