What causes breast cancer in people under 40 if there is no family history of the disease and no smoking involved?

We Don't Know: While we have made strides on the treatment end, we still struggle to understand the "why". What we do know is that 85-90% of cases occur in people with no known hereditary risk factors. We also see links with high (unopposed) estrogen states (early menarche, late menopause, no children or late-in-life, obesity).

Unknown: There are some increased breast cancer risks associated with obesity, hormone medication use, early age at first menstruation, late age at first childbirth, etc. Only about 15% of breast cancers occur because of genetic inheritance.