At the smallest level, breast cancer begins with changes inside individual cells. Normal cells follow a life cycle: they grow, divide, and then die when they are no longer needed. This process keeps the body balanced and functioning properly.

Cancer cells behave differently. Due to changes inside the cell, they lose the ability to control their growth. They continue to divide rapidly and do not die when they should.

These abnormal cells can also ignore signals from the body and may not function like healthy cells. Over time, the buildup of these uncontrolled cells leads to the formation of a tumor and can eventually allow the cancer to spread to other parts of the body.